Light-emitting element, light-emitting device, lighting device, and electronic device

ABSTRACT

An object is to provide a light-emitting element which exhibits light emission with high luminance and can be driven at low voltage. Another object is to provide a light-emitting device or an electronic device with reduced power consumption. Between an anode and a cathode, n (n is a natural number of two or more) EL layers are provided, where between a first EL layer and a second EL layer, a first layer containing any of an alkali metal, an alkaline earth metal, a rare earth metal, an alkali metal compound, an alkaline earth metal compound, and a rare earth metal compound, a second layer containing a material having a high electron-transporting property in contact with the first layer, and a region containing a material having a high hole-transporting property and an acceptor material in contact with the second layer are provided in this order from the anode side.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention hereinafter disclosed relates to a light-emittingelement having a light-emitting layer between a pair of electrodes. Inaddition, the present invention relates to a light-emitting device inwhich the light-emitting element is used and a lighting device and anelectronic device in each of which the light-emitting device is used.

2. Description of the Related Art

In recent years, a light-emitting element in which a light-emittingorganic compound or a light-emitting inorganic compound is used as alight-emitting material has been actively developed. In particular, alight-emitting element called an electroluminescence (hereinafter, EL)element has a simple structure in which a light-emitting layercontaining a light-emitting material is provided between electrodes, andhas attracted attention as a next-generation flat panel display elementbecause of its characteristics such as thinness, lightweight, highresponse speed, and direct current low voltage driving. In addition, adisplay in which such a light-emitting element is used has a featurethat it is excellent in contrast and image quality and has a wideviewing angle. Moreover, such a light-emitting element is a plane lightsource; therefore, application of such a light-emitting element isconsidered as a light source such as a backlight of a liquid crystaldisplay and lighting.

Current is applied to a light-emitting layer provided between a pair ofelectrodes in a light-emitting element to excite a light-emittingmaterial contained in the light-emitting layer, whereby a predeterminedemission color can be obtained. Supplying a large amount of current tothe light-emitting layer is considered in order to increase emissionluminance of such a light-emitting element; however, such a methodhinders reduction of power consumption. In addition, applying a largeamount of current also leads to acceleration of deterioration of thelight-emitting element.

Hence, a light-emitting element is proposed whose emission luminance isincreased by stacking a plurality of light-emitting layers and applyingcurrent which has the same current density as current applied in thecase of a light-emitting element having a single light-emitting layer(for example, Patent Document 1).

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    3933591

In Patent Document 1, a light-emitting element is proposed in which aplurality of light-emitting units (hereinafter in this specification,the light-emitting unit is also referred to as an EL layer) are providedand the light-emitting units are separated by a charge production layer.More specifically, a light-emitting element is proposed in which acharge production layer formed of vanadium pentoxide is provided over ametal-doped layer functioning as an electron-injecting layer of a firstlight-emitting unit, and further a second light-emitting unit is stackedover the metal-doped layer with the charge production layer interposedtherebetween. However, in the light-emitting element having such astructure, mutual interaction occurs between the metal-doped layer andthe charge-generation layer formed of the oxide at their interface andthe interface has a high electric field; thus, high voltage is neededfor driving the light-emitting element, unfortunately.

In view of the above problem, it is an object to provide alight-emitting element which exhibits light emission with high luminanceand can be driven at low voltage. In addition, it is another object toprovide a light-emitting device or an electronic device with reducedpower consumption.

SUMMARY OF THE INVENTION

One embodiment disclosed in this specification is a light-emittingelement having a structure in which n (n is a natural number of two ormore) EL layers are provided between an anode and a cathode, wherebetween m-th (m is a natural number, 1≦m≦−n−1) EL layer and (m+1)-th ELlayer, a first layer containing any of an alkali metal, an alkalineearth metal, a rare earth metal, an alkali metal compound, an alkalineearth metal compound, and a rare earth metal compound, a second layercontaining a material having a high electron-transporting property incontact with the first layer, and a region containing a material havinga high hole-transporting property and an acceptor material in contactwith the second layer are provided in this order from the anode side.

Another embodiment disclosed in this specification is a light-emittingelement having a structure in which n (n is a natural number of two ormore) EL layers are provided between an anode and a cathode, wherebetween m-th (m is a natural number, 1≦m≦−n−1) EL layer and (m+1)-th ELlayer, a first layer containing a material having a highelectron-transporting property and a donor material, a second layercontaining a material having a high electron-transporting property incontact with the first layer, and a region containing a material havinga high hole-transporting property and an acceptor material in contactwith the second layer are provided in this order from the anode side.

In addition, in the above first layer containing the material having ahigh electron-transporting property and the donor material, the donormaterial may be added so that the mass ratio of the donor material tothe material having a high electron-transporting property is from0.001:1 to 0.1:1. Moreover, the donor material is preferably an alkalimetal, an alkaline earth metal, a rare earth metal, an alkali metalcompound, an alkaline earth metal compound, or a rare earth metalcompound.

Furthermore, in the above structure, the region containing the materialhaving a high hole-transporting property and the acceptor material is aregion to which the acceptor material is added so that the mass ratio ofthe acceptor material to the material having a high hole-transportingproperty is from 0.1:1 to 4.0:1. Of carriers generated in the region,holes are injected into (m+1)-th EL layer and electrons move to thesecond layer.

Furthermore, in the above structure, the region containing the materialhaving a high hole-transporting property and the acceptor material maybe a region in which a layer containing the material having a highhole-transporting property and a layer containing the acceptor materialare stacked.

Furthermore, in the above structure, as the material having a highelectron-transporting property that is contained in the second layer, amaterial whose LUMO level is preferably greater than or equal to −5.0eV, more preferably greater than or equal to −5.0 eV and less than orequal to −3.0 eV is preferably used.

Furthermore, with the light-emitting element having the above structure,low driving voltage can be realized; therefore, low power consumption ofa light-emitting device (e.g., an image display device or alight-emitting device) in which the light-emitting element is used canbe realized. Thus, a light-emitting device in which the light-emittingelement having the above structure is used and a lighting device and anelectronic device in each of which the light-emitting device is used arealso included as one embodiment of the present invention.

With the above structure, at least one of the above objects is achieved.

Note that the light-emitting device in this specification includes, inits category, electronic devices such as an image display device and alighting device in each of which a light-emitting element is used.Further, the category of the light-emitting device includes a moduleincluding a light-emitting element attached with a connector such as amodule attached with an anisotropic conductive film, TAB (tape automatedbonding) tape, or a TCP (tape carrier package); a module in which thetop of the TAB tape or the TCP is provided with a printed wire board; ora module in which an IC (integrated circuit) is directly mounted on alight-emitting element by COG (chip on glass); and the like.

Note that the ordinal numbers such as “first” and “second” in thisspecification are used for convenience and do not denote the order ofsteps and the stacking order of layers. In addition, the ordinal numbersin this specification do not denote particular names which specify theinvention.

A light-emitting element which has a plurality of light-emitting layersand can be driven at low voltage can be provided.

In addition, by manufacture of a light-emitting device using theabove-described light-emitting element, the light-emitting device canhave low power consumption. Furthermore, such a light-emitting device isapplied to a lighting device and an electronic device, whereby thelighting device and the electronic device can have low powerconsumption.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1A illustrates an example of an element structure of alight-emitting element and FIG. 1B is a band diagram thereof;

FIG. 2A illustrates an example of an element structure of alight-emitting element and FIG. 2B is a band diagram thereof;

FIG. 3A illustrates an example of an element structure of alight-emitting element and FIG. 3B is a band diagram thereof;

FIGS. 4A and 4B are views each illustrating an element structure of alight-emitting element;

FIGS. 5A to 5C are each a cross-sectional view of an active matrixlight-emitting device;

FIGS. 6A and 6B are views illustrating a passive matrix light-emittingdevice;

FIGS. 7A to 7E are diagrams each illustrating an electronic device;

FIG. 8 is a diagram illustrating lighting devices;

FIGS. 9A and 9B are views illustrating structures of a light-emittingelement and a reference light-emitting element of Examples;

FIG. 10 is a graph showing characteristics of light-emitting elements ofExample 1;

FIG. 11 is a graph showing characteristics of the light-emittingelements of Example 1;

FIGS. 12A and 12B are views illustrating structures of a light-emittingelement and a reference light-emitting element of Examples;

FIG. 13 is a graph showing characteristics of light-emitting elements ofExample 2;

FIG. 14 is a graph showing characteristics of the light-emittingelements of Example 2;

FIG. 15 is a graph showing characteristics of light-emitting elements ofExample 3;

FIG. 16 is a graph showing characteristics of the light-emittingelements of Example 3;

FIG. 17 is a graph showing characteristics of light-emitting elements ofExample 4;

FIG. 18 is a graph showing characteristics of the light-emittingelements of Example 4;

FIG. 19 is a graph showing characteristics of light-emitting elements ofExample 5;

FIG. 20 is a graph showing characteristics of the light-emittingelements of Example 5;

FIG. 21 is a view illustrating a structure of a reference light-emittingelement of Example 6;

FIG. 22 is a graph showing characteristics of light-emitting elements ofExample 6;

FIG. 23 is a graph showing characteristics of the light-emittingelements of Example 6;

FIG. 24A is a view illustrating an example of a structure of alight-emitting element and FIG. 24B is a graph showing emission spectraof the light-emitting element;

FIG. 25 is a graph showing characteristics of light-emitting elements ofExample 7;

FIG. 26 is a graph showing characteristics of the light-emittingelements of Example 7;

FIG. 27 is a graph showing characteristics of light-emitting elements ofExample 8;

FIG. 28 is a graph showing characteristics of the light-emittingelements of Example 8;

FIG. 29 is a graph showing characteristics of the light-emittingelements of Example 8;

FIG. 30 is a graph showing characteristics of light-emitting elements ofExample 9;

FIG. 31 is a graph showing characteristics of the light-emittingelements of Example 9; and

FIG. 32 is a graph showing characteristics of the light-emittingelements of Example 9.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments and examples of the present invention disclosedwill be described with reference to the accompanying drawings. Note thatit is easily understood by those skilled in the art that the presentinvention disclosed can be carried out in many different modes, and themodes and details disclosed herein can be modified in various wayswithout departing from the spirit and scope of the present invention.Therefore, the present invention disclosed should not be construed asbeing limited to the description below of embodiments and examples. Inthe drawings for explaining the embodiments and examples, the same partsor parts having a similar function are denoted with the same referencenumerals, and description of such parts is not repeated.

Embodiment 1

In Embodiment 1, one embodiment of a light-emitting element will bedescribed with reference to FIGS. 1A and 1B.

In an element structure illustrated in FIG. 1A, a first EL layer 103 anda second EL layer 107 each including a light-emitting region aresandwiched between a pair of electrodes (an anode 101 and a cathode102), and between the first EL layer 103 and the second EL layer 107, anelectron-injecting buffer 104, an electron-relay layer 105, and a chargeproduction region 106 are stacked in this order from the anode 101 side.

The charge production region 106 is a region containing a materialhaving a high hole-transporting property and an acceptor material, whereholes and electrons that are carriers of the light-emitting element areproduced. The holes produced in the charge production region 106 move tothe second EL layer 107 while the electrons move to the electron-relaylayer 105. Further, since the electron-relay layer 105′ has a highelectron-transporting property, it can rapidly transport the electronsto the electron-injecting buffer 104. Moreover, since theelectron-injecting buffer 104 can reduce injection barriers when theelectrons are injected into the first EL layer 103, it can increase theefficiency of electron injection into the first EL layer 103.

A material having a high electron-injecting property can be used for theelectron-injecting buffer 104: for example, an alkali metal, an alkalineearth metal, a rare earth metal, a compound of the above metal (e.g., analkali metal compound (an oxide such as lithium oxide, a halide, andcarbonate such as lithium carbonate or cesium carbonate), an alkalineearth metal compound (e.g., an oxide, a halide, and carbonate), and arare earth metal compound (e.g., an oxide, a halide, and carbonate).Alternatively, the electron-injecting buffer 104 may contain a materialhaving a high electron-transporting property and a donor material.

FIG. 1B is a band diagram of the element structure of FIG. 1A. In FIG.1B, reference numeral 111 denotes the Fermi level of the anode 101; 112,the Fermi level of the cathode 102; 113, the lowest unoccupied molecularorbital (LUMO) level of the first EL layer 103; 114, the LUMO level ofthe electron-relay layer 105; 115, the acceptor level of an acceptor inthe charge production region 106; and 116, the LUMO level of the secondEL layer 107.

In FIG. 1B, the holes injected from the anode 101 are injected into thefirst EL layer 103. On the other hand, the electrons generated in thecharge production region 106 move to the electron-relay layer 105, andthen injected into the first EL layer 103 through the electron-injectingbuffer 104, and recombined with the holes, whereby light is emitted. Inaddition, the holes generated in the charge production region 106 moveto the second EL layer 107 and recombined with the electrons injectedfrom the cathode 102 in the second EL layer 107, whereby light isemitted.

In the light-emitting element described in this embodiment, since theelectron-relay layer 105 functions as a layer for sufficiently injectingelectrons generated in the charge production region 106 into the firstEL layer 103, the electron-relay layer 105 is preferably formed using amaterial whose LUMO level is a level between the acceptor level of theacceptor in the charge production region 106 and the LUMO level of thefirst EL layer 103. Specifically, a material whose LUMO level is greaterthan or equal to about −5.0 eV is preferably used, and a material whoseLUMO level is greater than or equal to −5.0 eV and less than or equal to−3.0 eV is more preferably used.

The acceptor material contained in the charge production region 106 hasa strong acceptor property, and the material having a highelectron-injecting property or the donor material contained in theelectron-injecting buffer 104 has a strong donor property; therefore,when the charge production region 106 and the electron-injecting buffer104 are in contact with each other, electrons are donated and acceptedat the interface between the charge production region 106 and theelectron-injecting buffer 104, which leads to an increase in drivingvoltage of the light-emitting element. In addition, the driving voltageof the light-emitting element is likely to be increased when PN junctionis formed at the interface where the charge production region 106 andthe electron-injecting buffer 104 are in contact with each other.However, in the light-emitting element described in this embodiment, thecharge production region 106 and the electron-injecting buffer 104 canbe prevented from being in contact with each other by the electron-relaylayer 105; thus, the acceptor material contained in the chargeproduction region 106 and the material having a high electron-injectingproperty or the donor material contained in the electron-injectingbuffer 104 can be prevented from interacting with each other by theelectron-relay layer 105. In addition, the electron-relay layer 105 isformed using a material whose LUMO level falls within theabove-described range, whereby a high electric field of the interfacebetween the electron-relay layer 105 and the electron-injecting buffer104 is suppressed, and the electrons generated in the charge productionregion 106 can be efficiently injected into the first EL layer 103.

Further, as illustrated in the band diagram of FIG. 1B, the electronsthat have moved to the electron-relay layer 105 from the chargeproduction region 106 are easily injected into the LUMO level 113 of thefirst EL layer 103 because of reduced injection barrier due to theelectron-injecting buffer 104. Note that the holes generated in thecharge production region 106 move to the second EL layer 107.

Next, the material that can be used for the above-describedlight-emitting element is specifically described.

The anode 101 is preferably formed using a metal, an alloy, anelectrically-conductive compound, a mixture of these materials, or thelike, having a high work function (specifically, a work function ofgreater than or equal to 4.0 eV). Specifically, indium tin oxide (ITO),indium tin oxide containing silicon or silicon oxide, indium zinc oxide(IZO), indium oxide containing tungsten oxide and zinc oxide, and thelike can be given, for example.

Films of these conductive metal oxides are usually formed by sputteringmethod. Alternatively, the films may be formed by application of asol-gel method or the like. For example, a film of indium oxide-zincoxide (IZO) can be formed by a sputtering method using a target in whichzinc oxide is added to indium oxide at 1 wt % to 20 wt %. Indium oxidecontaining tungsten oxide and zinc oxide can be formed by a sputteringmethod using a target in which tungsten oxide and zinc oxide are addedto indium oxide at 0.5 wt % to 5 wt % and 0.1 wt % to 1 wt %,respectively.

Besides, as a material used for the anode 101, the following can begiven: gold (Au), platinum (Pt), nickel (Ni), tungsten (W), chromium(Cr), molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu), palladium(Pd), titanium (Ti), nitride of a metal material (e.g., titaniumnitride), molybdenum oxide, vanadium oxide, ruthenium oxide, tungstenoxide, manganese oxide, titanium oxide, and the like. Alternatively, aconductive polymer such aspoly(3,4-ethylenedioxythiophene)/poly(styrenesulfonic acid) (PEDOT/PSS)or polyaniline/poly(styrenesulfonic acid) (PAni/PSS) may be used. Notethat, in the case where a charge production region is provided incontact with the anode 101 as part of the first EL layer 103, a varietyof conductive materials such as Al and Ag can be used for the anode 101regardless of the magnitude of their work functions.

The cathode 102 is preferably formed using a metal, an alloy, anelectrically-conductive compound, a mixture of these materials, or thelike, having a low work function (specifically, a work function of lessthan or equal to 3.8 eV). As specific examples of such a cathodematerial, the following can be given: an element that belongs to Group 1or 2 of the periodic table, that is, an alkali metal such as lithium(Li) or cesium (Cs), an alkaline earth metal such as magnesium (Mg),calcium (Ca), or strontium (Sr), an alloy containing these (such as anMgAg alloy or an AlLi alloy), a rare-earth metal such as europium (Eu)or ytterbium (Yb), an alloy containing these, and the like. Note that afilm of an alkali metal, an alkaline earth metal, or an alloy thereofcan be formed by a vacuum evaporation method. Alternatively, an alloycontaining an alkali metal or an alkaline earth metal can be formed by asputtering method. Further alternatively, a film can be formed usingsilver paste or the like by an ink jet method or the like.

Alternatively, the cathode 102 can be formed using a stack of a thinfilm of an alkali metal compound, an alkaline earth metal compound, or arare earth metal compound (e.g., lithium fluoride (LiF), lithium oxide(LiOx), cesium fluoride (CsF), calcium fluoride (CaF₂), or erbiumfluoride (ErF₃)) and a film of a metal such as aluminum. Note that, inthe case where the charge production region is provided in contact withthe cathode 102 as part of the second EL layer 107, a variety ofconductive materials such as Al, Ag, ITO, and indium oxide-tin oxidecontaining silicon or silicon oxide can be used for the cathode 102regardless of the magnitude of their work functions.

Note that in the light-emitting element described in this embodiment, atleast one of the anode and the cathode may have a light-transmittingproperty. The light-transmitting property can be ensured with use of atransparent electrode such as ITO, or reduction in the thickness of anelectrode.

The first EL layer 103 and the second EL layer 107 each may include atleast a light-emitting layer, and may also have a structure in which alight-emitting layer and layers other than the light-emitting layer arestacked. Note that the light-emitting layer included in the first ELlayer 103 may be different from the light-emitting layer included in thesecond EL layer 107. Alternatively, the first EL layer 103 and thesecond EL layer 107 may independently have a structure in which alight-emitting layer and layers other than the light-emitting layer arestacked. As the layers other than the light-emitting layer, there arelayers formed of a material having a high hole-injecting property, amaterial having a high hole-transporting property, a material having ahigh electron-transporting property, a material having a highelectron-injecting property, a material having a bipolar property (amaterial having high electron-and-hole-transporting properties), and thelike. Specifically, a hole-injecting layer, a hole-transporting layer, ahole-blocking layer, a light-emitting layer, an electron-transportinglayer, an electron-injecting layer, and the like are given, and they canbe combined as appropriate and stacked from the anode side. Furthermore,a charge production region can be provided in a portion of the first ELlayer, which is on the side where the first EL layer 103 is in contactwith the anode 101.

A material which is used for forming each of the above-described layersincluded in the EL layer is specifically described.

The hole-injecting layer is a layer containing a material having a highhole-injecting property. As the material having a high hole-injectingproperty, for example, molybdenum oxide, vanadium oxide, rutheniumoxide, tungsten oxide, manganese oxide, or the like can be used.Besides, a phthalocyanine-based compound such as phthalocyanine(abbreviation: H₂Pc) or copper phthalocyanine (abbreviation: CuPc), ahigh molecule such aspoly(3,4-ethylenedioxythiophene)/poly(styrenesulfonic acid) (PEDOT/PSS),or the like can also be used for forming the hole-injecting layer.

The hole-transporting layer is a layer containing a material having ahigh hole-transporting property. As the material having a highhole-transporting property, the following can be given, for example:aromatic amine compounds such as4,4′-bis[N-(1-naphtyl)-N-phenylamino]biphenyl (abbreviation: NPB orα-NPD),N,N-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), 4,4′,4″-tris(carbazol-9-yl)triphenylamine(abbreviation: TCTA), 4,4′,4″-tris(N,N-diphenylamino)triphenylamine(abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA), and4,4′-bis[N-(spiro-9,9′-bifluorene-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB);3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1);3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2),3-[N-(1-naphtyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1), and the like. Alternatively, the followingcarbazole derivative can be used: 4,4′-di(N-carbazolyl)biphenyl(abbreviation: CBP), 1,3,5-tris[4-(N-carbazolyl)phenyl]benzene(abbreviation: TCPB), and 9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: CzPA). The materials listed here are mainly materialshaving a hole mobility of greater than or equal to 10⁻⁶ cm²/Vs. However,materials other than those can also be used as long as they have ahole-transporting property higher than an electron-transportingproperty. The layer containing a material having a highhole-transporting property is not limited to a single layer, and may bea stack of two or more layers each containing the above-describedmaterial.

In addition to the above materials, a high molecular compound such aspoly(N-vinylcarbazole) (abbreviation: PVK), poly(4-vinyltriphenylamine)(abbreviation: PVTPA),poly[N-(4-{N′-[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide](abbreviation:PTPDMA), or poly[N,N-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine](abbreviation: Poly-TPD) can be used for the hole-transporting layer.

The light-emitting layer is a layer containing a light-emittingmaterial. As the light-emitting material, the following fluorescentcompound can be used, for example:N,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine(abbreviation:YGA2S),4-(9H-carbazol-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine(abbreviation: YGAPA),4-(9H-carbazol-9-yl)-4′-(9,10-diphenyl-2-anthryl)triphenylamine(abbreviation: 2YGAPPA),N,9-diphenyl-N-[4-(10-phenyl-9-antryl)phenyl]-9H-carbazol-3-amine(abbreviation: PCAPA), perylene, 2,5,8,11-tetra-tert-butylperylene(abbreviation: TBP),4-(10-phenyl-9-anthryl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBAPA),N,N″-(2-tert-butylanthracene-9,10-diyldi-4,1-phenylene)bis[N,N′,N′-triphenyl-1,4-phenylenediamine](abbreviation: DPABPA),N,9-diphenyl-N-[4-(9,10-diphenyl-2-anthryl)phenyl]-9H-carbazole-3-amine(abbreviation: 2PCAPPA),N-[4-(9,10-diphenyl-2-anthryl)phenyl]-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPAPPA),N,N,N′,N′,N″,N″,N′″,N′″-octaphenyldibenzo[g,p]chrysene-2,7,10,15-tetraamine(abbreviation: DBC1), coumarin 30,N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA),N49,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCABPhA),N-(9,10-diphenyl-2-anthryl)-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPAPA),N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPABPhA),9,10-bis(1,1′-biphenyl-2-yl)-N-[4-(9H-carbazol-9-yl)phenyl]-N-phenylanthracen-2-amine(abbreviation: 2YGABPhA), N,N,9-triphenylanthracen-9-amine(abbreviation: DPhAPhA), coumarin 545T, N,N′-diphenylquinacridone(abbreviation: DPQd), rubrene,5,12-bis(1,1′-biphenyl-4-yl)-6,11-diphenyltetracene (abbreviation: BPT),2-(2-{2-[4-(dimethylamino)phenyl]ethenyl}-6-methyl-4H-pyran-4-ylidene)propanedinitrile(abbreviation: DCM1),2-{2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl(ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCM2),N,N,N′,N′-tetrakis(4-methylphenyl)tetracene-5,11-diamine (abbreviation:p-mPhTD),7,14-diphenyl-N,N,N′,N′-tetrakis(4-methylphenyl)acenaphtho[1,2-a]fluoranthene-3,10-diamine(abbreviation: p-mPhAFD),2-{2-isopropyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCJTI),2-{2-tert-butyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCJTB),2-(2,6-bis{2-[4-(dimethylamino)phenyl]ethenyl}-4H-pyran-4-ylidene)propanedinitrile(abbreviation: BisDCM),2-{2,6-bis[2-(8-methoxy-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: BisDCJTM), and the like.

Alternatively, as the light-emitting material, the followingphosphorescent compound can be used, for example:bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)tetrakis(1-pyrazolyl)borate(abbreviation: FIr6),bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)picolinate(abbreviation: FIrpic),bis[2-(3′,5′-bistrifluoromethylphenyl)pyridinato-N,C^(2′)]iridium(III)picolinate(abbreviation: Ir(CF₃ ppy)₂(pic)),bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)acetylacetonate (abbreviation: FIracac),tris(2-phenylpyridinato-N,C^(2′))iridium(III) (abbreviation: Ir(ppy)₃),bis(2-phenylpyridinato)iridium(III)acetylacetonato (abbreviation:Ir(ppy)₂(acac)), bis(benzo[h]quinolinato)iridium(III)acetylacetonate(abbreviation: Ir(bzq)₂(acac)),bis(2,4-diphenyl-1,3-oxazolato-N,C^(2′))iridium(III)acetylacetonate(abbreviation: Ir(dpo)₂(acac)),bis[2-(4′-perfluorophenylphenyl)pyridinato]iridium(III)acetylacetonate(abbreviation: Ir(p-PF-ph)₂(acac)),bis(2-phenylbenzothiazolato-N,C^(2′))iridium(III)acetylacetonate(abbreviation: Ir(bt)₂(acac)),bis[2-(2′-benzo[4,5-α]thienyl)pyridinato-N,C^(3′)]iridium(III)acetylacetonate (abbreviation: Ir(btp)₂(acac)),bis(1-phenylisoquinolinato-N,C^(2′))iridium(III) acetylacetonate(abbreviation: Ir(piq)₂(acac)),(acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III)(abbreviation: Ir(Fdpq)₂(acac)),(acetylacetonato)bis(2,3,5-triphenylpyrazinato)iridium(III)(abbreviation: Ir(tppr)₂(acac)),2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum(II)(abbreviation: PtOEP),tris(acetylacetonato)(monophenanthroline)terbium(III) (abbreviation:Tb(acac)₃(Phen)),tris(1,3-diphenyl-1,3-propanedionato)(monophenanthroline)europium(III)(abbreviation: Eu(DBM)₃(Phen)),tris[1-(2-thenoyl)-3,3,3-trifluoroacetonato](monophenanthroline)europium(III)(abbreviation: Eu(TTA)₃(Phen)), and the like.

Note that those light-emitting materials are preferably dispersed in ahost material to be used. As the host material, for example, thefollowing can be used: an aromatic amine compound such as NPB(abbreviation), TPD (abbreviation), TCTA (abbreviation), TDATA(abbreviation), MTDATA (abbreviation), or BSPB (abbreviation); acarbazole derivative such as PCzPCA1 (abbreviation), PCzPCA2(abbreviation), PCzPCN1 (abbreviation), CBP (abbreviation), TCPB(abbreviation), CzPA (abbreviation), or4-(1-naphthyl)-4′-(9-phenyl-9H-carbazol-3-yl)-triphenylamine(abbreviation: PCBANB); a material having a high hole-transportingproperty which contains a high molecular compound, such as PVK(abbreviation), PVTPA (abbreviation), PTPDMA (abbreviation), or Poly-TPD(abbreviation); a metal complex having a quinoline skeleton or abenzoquinoline skeleton, such as tris(8-quinolinolato)aluminum(abbreviation: Alq), tris (4-methyl-8-quinolinolato)aluminum(abbreviation: Almq₃), bis(10-hydroxybenzo[h]-quinolinato)beryllium(abbreviation: BeBq₂), orbis(2-methyl-8-quinolinolato)(4-phenylphenolate)aluminum (abbreviation:BAlq); a metal complex having an oxazole-based or thiazole-based ligand,such as bis[2-(2-hydroxyphenyl)benzoxazolato]zinc (abbreviation:Zn(BOX)₂) or bis[2-(2-hydroxyphenyl)-benzothiazolato]zinc (abbreviation:Zn(BTZ)₂); or a material having a high electron-transporting property,such as 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole(abbreviation: PBD),1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),9-[4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl]carbazole (abbreviation:CO11), 3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ), bathophenanthroline (abbreviation: BPhen),bathocuproine (abbreviation: BCP),poly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)](abbreviation: PF-Py), orpoly[(9,9-dioctyllfluorene-2,7-diyl)-co-(2,2′-bipyridine-6,6′-diyl)](abbreviation: PF-BPy).

The electron-transporting layer is a layer containing a material havinga high electron-transporting property. As the material having a highelectron-transporting property, for example, a metal complex having aquinoline skeleton or a benzoquinoline skeleton, such as Alq(abbreviation), Almq₃ (abbreviation), BeBq₂ (abbreviation), or BAlq(abbreviation) can be used. In addition to the above, a metal complexhaving an oxazole-based or thiazole-based ligand, such as Zn(BOX)₂(abbreviation) or Zn(BTZ)₂ (abbreviation) can also be used. Furthermore,in addition to the above metal complexes, PBD (abbreviation), OXD-7(abbreviation), CO11 (abbreviation), TAZ (abbreviation), BPhen(abbreviation), BCP (abbreviation), or the like can also be used. Thematerials listed here are mainly materials having an electron mobilityof greater than or equal to 10⁻⁶ cm²/Vs. Note that materials other thanthose may be used as long as they have an electron-transporting propertyhigher than a hole-transporting property. Furthermore, theelectron-transporting layer may have a structure in which two or morelayers formed of the above materials are stacked, without limitation toa single-layer structure.

In addition to the above materials, a high molecular compound such asPF-Py (abbreviation) or PF-BPy (abbreviation) can be used for theelectron-transporting layer.

The electron-injecting layer is a layer containing a material having ahigh electron-injecting property. As the material having a highelectron-injecting property, the following can be given: an alkali metalor an alkaline earth metal such as lithium fluoride (LiF), cesiumfluoride (CsF), and calcium fluoride (CaF₂), and a compound thereof.Alternatively, a layer containing a material having anelectron-transporting property and an alkali metal, an alkaline earthmetal, or a compound thereof (e.g., Alq containing magnesium (Mg)) canbe used. Such a structure makes it possible to increase the efficiencyof injection of electrons from the cathode 102.

In the case where a charge production region is provided in the first ELlayer 103 or the second EL layer 107, the charge production region is aregion that contains a material having a high hole-transporting propertyand an acceptor material. The charge production region may not onlycontain a material having a high hole-transporting property and anacceptor material in the same film but also includes a stacked layer ofa layer containing a material having a high hole-transporting propertyand a layer containing an acceptor material. However, in the case of thestacked-layer structure provided on the anode side, the layer containingan acceptor material is in contact with the anode 101, while in the caseof the stacked-layer structure provided on the cathode side, the layercontaining a material having a high hole-transporting property is incontact with the cathode 102.

The charge production region is formed in the first EL layer 103 or thesecond EL layer 107, whereby the anode 101 or the cathode 102 can beformed without consideration of a work function of a material forforming an electrode.

As the acceptor material that is used for the charge production region,a transition metal oxide and an oxide of a metal belonging to Group 4 or8 of the periodic table can be given. Specifically, molybdenum oxide isparticularly preferable. Note that molybdenum oxide has a lowhygroscopic property.

As the material having a high hole-transporting property used for thecharge production region, any of a variety of organic compounds such asan aromatic amine compound, a carbazole derivative, an aromatichydrocarbon, and a high molecular compound (such as an oligomer, adendrimer, or a polymer) can be used. Specifically, a material having ahole mobility of greater than or equal to 10⁻⁶ cm²/Vs is preferable.However, materials other than those can also be used as long as theyhave a hole-transporting property higher than an electron-transportingproperty.

Note that layers formed of the above-described materials are stacked inappropriate combination, whereby the first EL layer 103 or the second ELlayer 107 can be formed. Further, as a formation method of the first ELlayer 103 or the second EL layer 107, any of a variety of methods (e.g.,a dry process and a wet process) can be selected as appropriatedepending on a material to be used. For example, a vacuum evaporationmethod, an ink-jet method, a spin coating method, or the like can beused. Note that a different formation method may be employed for eachlayer.

Further, between the first EL layer 103 and the second EL layer 107, theelectron-injecting buffer 104, the electron-relay layer 105, and thecharge production region 106 are provided in this order from the anode101 side. The charge production region 106 is formed in contact with thesecond EL layer 107, the electron-relay layer 105 is formed in contactwith the charge production region 106, and the electron-injecting buffer104 is formed in contact with and between the electron-relay layer 105and the first EL layer 103.

The charge production region 106 is a region that contains a materialhaving a high hole-transporting property and an acceptor material. Notethat the charge production region 106 can be formed using a materialsimilar to the above-described material used for the charge productionregion that can be formed in part of the first EL layer 103 or thesecond EL layer 107, whereby the charge production region 106 can have asimilar structure to the charge production region. Therefore, the chargeproduction region 106 can not only contain a material having a highhole-transporting property and an acceptor material in the same film butalso include a stacked layer of a layer containing a material having ahigh hole-transporting property and a layer containing an acceptormaterial. Note that in the case of the stacked layer, the layercontaining a material having a high hole-transporting property is incontact with the second EL layer 107.

Note that the acceptor material is preferably added to the chargeproduction region 106 so that the mass ratio of the acceptor material tothe material having a high hole-transporting property is from 0.1:1 to4.0:1.

The electron-relay layer 105 is a layer that can quickly receiveelectrons drawn out by the acceptor material in the charge productionregion 106. Therefore, the electron-relay layer 105 is a layer thatcontains a material having a high electron-transporting property and ispreferably formed using a material having a LUMO level between theacceptor level of the acceptor in the charge production region 106 andthe LUMO level of the first EL layer 103. Specifically, a material whoseLUMO level is greater than or equal to about −5.0 eV is preferably used,and a material whose LUMO level is greater than or equal to about −5.0eV and less than or equal to −3.0 eV is more preferably used. As thematerial used for the electron-relay layer 105, for example, a perylenederivative and a nitrogen-containing condensed aromatic compound can begiven. Note that a nitrogen-containing condensed aromatic compound ispreferably used for the electron-relay layer 105 because of itsstability. Furthermore, of nitrogen-containing condensed aromaticcompounds, a compound having an electron-withdrawing group such as acyano group or a fluoro group is preferably used, in which caseelectrons are easily received in the electron-relay layer 105.

As specific examples of the perylene derivative, the following can begiven: 3,4,9,10-perylenetetracarboxylicdianhydride (PTCDA),3,4,9,10-perylenetetracarboxylic-bis-benzimidazole (PTCBI),N,N′-dioctyl-3,4,9,10-perylenetetracarboxylic diimide (PTCDI-C8H),N,N′-dihexyl-3,4,9,10-perylenetetracarboxylicdiimide (HexPTC), and thelike.

As specific examples of the nitrogen-containing condensed aromaticcompound, the following can be given:pirazino[2,3-f][1,10]phenanthroline-2,3-dicarbonitrile (abbreviation:PPDN), 2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene(abbreviation: HAT(CN)₆), 2,3-diphenylpyrido[2,3-b]pyrazine(abbreviation: 2PYPR), 2,3-bis(4-fluorophenyl)pyrido[2,3-b]pyrazine(abbreviation: F2PYPR), and the like. Besides, perfluoropentacene,7,7,8,8-tetracyanoquinodimethane (abbreviation: TCNQ),1,4,5,8-naphthalenetetracarboxylicdianhydride (abbreviation: NTCDA),copper hexadecafluoro phthalocyanine (abbreviation: F₁₆CuPc), N,N′-bis(2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctyl-1,4,5,8-naphthalenetetracarboxylicdiimide (abbreviation: NTCDI-C8F),3′,4′-dibutyl-5,5″-bis(dicyanomethylene)-5,5″-dihydro-2,2′:5′,2″-terthiophen)(abbreviation: DCMT), methanofullerene such as [6,6]-phenyl C₆₁ butyricacid methyl ester (abbreviation: PCBM), or the like can be used for theelectron-relay layer 105.

The electron-injecting buffer 104 is a layer that can inject theelectrons received by the electron-relay layer 105 into the first ELlayer 103. The provision of the electron-injecting buffer 104 makes itpossible to reduce the injection barrier between the charge productionregion 106 and the first EL layer 103; thus, the electrons generated inthe charge production region 106 can be easily injected into the firstEL layer 103.

A material having a high electron-injecting property can be used for theelectron-injecting buffer 104: for example, an alkali metal, an alkalineearth metal, a rare earth metal, a compound of the above metal (e.g., analkali metal compound (an oxide such as lithium oxide or the like, ahalide, and carbonate such as lithium carbonate or cesium carbonate), analkaline earth metal compound (e.g., an oxide, a halide, and carbonate),and a rare earth metal compound (e.g., an oxide, a halide, andcarbonate).

Further, in the case where the electron-injecting buffer 104 contains amaterial having a high electron-transporting property and a donormaterial, the donor material is preferably added so that the mass ratioof the donor material to the material having an electron-transportingproperty is from 0.001:1 to 0.1:1. Note that as the donor material, anorganic compound such as tetrathianaphthacene (abbreviation: TTN),nickelocene, or decamethylnickelocene can be used as well as an alkalimetal, an alkaline earth metal, a rare earth metal, a compound of theabove metal (e.g., an alkali metal compound (e.g., an oxide of lithiumoxide or the like, a halide, and carbonate such as lithium carbonate orcesium carbonate), an alkaline earth metal compound (e.g., an oxide, ahalide, and carbonate), and a rare earth metal compound (e.g., an oxide,a halide, and carbonate). Note that as the material having a highelectron-transporting property, a material similar to theabove-described material for the electron-transporting layer that can beformed in part of the first EL layer 103 can be used.

The light-emitting element described in this embodiment can bemanufactured by combination of the above-described materials. Althoughlight emission from the above-described light-emitting material can beobtained with this light-emitting element, a variety of emission colorscan be obtained by changing the type of the light-emitting material thatis used for the light-emitting layer. In addition, a plurality oflight-emitting materials of different colors are used as thelight-emitting material, whereby light emission having a broad spectrumor white light emission can also be obtained.

Note that, although the light-emitting element in which two EL layersare provided is described in this embodiment, the number of EL layers isnot limited to two, and may be three, for example. In the case where n(n is a natural number of two or more) EL layers are provided in alight-emitting element, an electron-injecting buffer, an electron-relaylayer, and a charge production region are stacked in this order from theanode side between m-th (m is a natural number, 1≦m≦n−1) EL layer and(m+1)-th EL layer, whereby an increase in the driving voltage of thelight-emitting element can be suppressed.

Further, the light-emitting element described in this embodiment can beformed over any of a variety of substrates. As the substrate, forexample, a substrate made of glass, plastic, a metal plate, metal foil,or the like can be used. In the case where light emission of thelight-emitting element is extracted from the substrate side, a substratehaving a light-transmitting property may be used. Note that as thesubstrate, a substrate other than the above may be used as long as itcan serve as a support in the manufacturing process of thelight-emitting element.

Note that a passive matrix light-emitting device in which bothelectrodes are formed in a grid pattern over the same substrate can bemanufactured with the structure of the light-emitting element describedin this embodiment. In addition, an active matrix light-emitting deviceincluding a light-emitting element which is electrically connected to athin film transistor (TFT) functioning as a switch, or the like and thedriving of which is controlled by the TFT can also be manufactured withthe structure of the light-emitting element described in thisembodiment. Note that the structure of the TFT is not particularlylimited. A staggered TFT or an inverted staggered TFT may be employed.In addition, a driver circuit formed with a TFT may be formed using ann-type TFT and a p-type TFT, or using either an n-type TFT or a p-typeTFT. Crystallinity of a semiconductor film used for the TFT is notparticularly limited, either. An amorphous semiconductor film may beused, or a crystalline semiconductor film may be used. Alternatively, asingle crystal semiconductor film or a microcrystalline semiconductormay be used. Further alternatively, an oxide semiconductor, for example,an oxide semiconductor containing indium, gallium, and zinc can be used.

Further, the light-emitting element described in this embodiment can beformed by any of a variety of methods regardless of whether it is a dryprocess (e.g., a vacuum evaporation method or a sputtering method) or awet process (e.g., an ink-jet method, a spin coating method, or anapplication method).

The element structure described in this embodiment is employed, wherebythe driving voltage of the light-emitting element can be less likely tobe adversely affected by the thickness of the charge production region106. Thus, an increase in the driving voltage of the light-emittingelement can be suppressed, and improvement of color purity by opticaladjustment can be realized.

In addition, when the element structure described in this embodiment isemployed, the electron-relay layer 105 is sandwiched between the chargeproduction region 106 and the electron-injecting buffer 104. In thiscase, a structure in which the acceptor contained in the chargeproduction region 106 and the material having a high electron-injectingproperty or the donor material contained in the electron-injectingbuffer 104 are less likely to interact, and thus their functions areless likely to be inhibited can be obtained. Thus, the light-emittingelement can be driven at low voltage.

Note that the structure described in this embodiment can be combinedwith any of the structures described in other embodiments asappropriate.

Embodiment 2

In Embodiment 2, an example of the light-emitting element included inthe basic structure described in Embodiment 1 will be described withreference to FIGS. 2A and 2B. Specifically, a case where theelectron-injecting buffer 104 has a single layer of an alkali metal, analkaline earth metal, a rare earth metal, or a compound thereof will bedescribed.

As illustrated in FIG. 2A, in a light-emitting element described in thisembodiment, the first EL layer 103 and the second EL layer 107 eachincluding a light-emitting region are sandwiched between a pair ofelectrodes (the anode 101 and the cathode 102), and between the first ELlayer 103 and the second EL layer 107, the electron-injecting buffer104, the electron-relay layer 105, and the charge production region 106are stacked in this order from the anode 101 side.

The anode 101, the cathode 102, the first EL layer 103, the second ELlayer 107, the charge production region 106, and the electron-relaylayer 105 in Embodiment 2 can be formed using materials similar to thosedescribed in Embodiment 1.

In this embodiment, as a material used for the electron-injecting buffer104, the following can be given: materials having a highelectron-injecting property, such as alkali metals such as lithium (Li)and cesium (Cs); alkaline earth metals such as magnesium (Mg), calcium(Ca), and strontium (Sr); rare earth metals such as europium (Eu) andytterbium (Yb); alkali metal compounds (e.g., an oxide of lithium oxideand the like, a halide, and carbonate such as lithium carbonate andcesium carbonate); alkaline earth metal compounds (e.g., an oxide, ahalide, and carbonate), and rare earth metal compounds (e.g., an oxide,a halide, and carbonate); and the like.

In the light-emitting element described in this embodiment, a singlelayer of any of the above-described metals or a compound thereof isprovided as the electron-injecting buffer 104. The electron-injectingbuffer 104 is formed to have a very small thickness (specifically, lessthan or equal to 1 nm) so that an increase in the driving voltage isprevented. Note that in this embodiment, the electron-transporting layer108 is preferably formed in contact with the electron-injecting buffer104 in the first EL layer 103 and that the electron-injecting buffer 104is proximately located at the interface between the electron-relay layer105 and the electron-transporting layer 108, which is part of the ELlayer 103. However, in the case where the electron-injecting buffer 104is formed over the electron-transporting layer 108 after theelectron-transporting layer 108 is formed, part of the material used forforming the electron-injecting buffer 104 can also exist in theelectron-transporting layer 108 that is part of the EL layer 103.

FIG. 2B is a band diagram of the element structure of FIG. 2A. In FIG.2B, the electron-injecting buffer 104 is provided at the interfacebetween the electron-relay layer 105 and the first EL layer 103, wherebythe injection barrier between the charge production region 106 and thefirst EL layer 103 can be reduced; thus, electrons generated in thecharge production region 106 can be easily injected into the first ELlayer 103. In addition, holes generated in the charge production region106 move to the second EL layer 107.

The structure of the electron-injecting buffer described in thisembodiment is employed, whereby the driving voltage of thelight-emitting element can be reduced in comparison with a structure ofan electron-injecting buffer described later in Embodiment 3 (that isformed by addition of a donor material to a material having a highelectron-transporting property). Note that in this embodiment, as thematerial having a high electron-injecting property in theelectron-injecting buffer 104, an alkali metal compound (e.g., an oxidesuch as lithium oxide, a halide, and carbonate such as lithium carbonateor cesium carbonate), an alkaline earth metal compound (e.g., an oxide,a halide, and carbonate), a rare earth metal compound (e.g., an oxide, ahalide, and carbonate), or the like is preferably used. Those materialshaving a high electron-injecting property are stable in the air, andtherefore provide high productivity and are suitable for massproduction.

Note that the structure described in this embodiment can be combinedwith any of the structures described in other embodiments asappropriate.

Embodiment 3

In Embodiment 3, an example of the light-emitting element included inthe basic structure described in Embodiment 1 will be described withreference to FIGS. 3A and 3B. Specifically, a case where theelectron-injecting buffer 104 of the light-emitting element described inEmbodiment 1 contains a material having a high electron-transportingproperty and a donor material.

As illustrated in FIG. 3A, in a light-emitting element described in thisembodiment, the first EL layer 103 and the second EL layer 107 eachincluding a light-emitting region are sandwiched between a pair ofelectrodes (the anode 101 and the cathode 102), and between the first ELlayer 103 and the second EL layer 107, the electron-injecting buffer104, the electron-relay layer 105, and the charge production region 106are stacked in this order from the anode 101 side. In addition, theelectron-injecting buffer 104 contains a material having a highelectron-transporting property and a donor material.

Note that, in this embodiment, the donor material is preferably added sothat the mass ratio of the donor material to the material having a highelectron-transporting property is from 0.001:1 to 0.1:1. Accordingly,the electron-injecting buffer 104 can have high film quality and highreactivity.

The anode 101, the cathode 102, the EL layer 103, the charge productionregion 106, and the electron-relay layer 105 in Embodiment 3 can beformed using materials similar to those described in Embodiment 1.

In this embodiment, as the material having a high electron-transportingproperty used for the electron-injecting buffer 104, the following canbe used, for example: a metal complex having a quinoline skeleton or abenzoquinoline skeleton, such as tris(8-quinolinolato)aluminum(abbreviation: Alq), tris(4-methyl-8-quinolinolato)aluminum(abbreviation: Almq₃), bis(10-hydroxybenzo[h]-quinolinato)beryllium(abbreviation: BeBq₂), orbis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum (abbreviation:BAlq), or the like. Alternatively, a metal complex having anoxazole-based or thiazole-based ligand, such asbis[2-(2-hydroxyphenyl)benzoxazolato]zinc (abbreviation: Zn(BOX)₂) orbis[2-(2-hydroxyphenyl)-benzothiazolato]zinc (abbreviation: Zn(BTZ)₂)can be used. Further alternatively, besides the metal complexes,2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),9-[4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl]carbazole (abbreviation:CO11), 3-(4-biphenylyl)-4-phenyl-5-(4-tert-biphenylyl)-1,2,4-triazole(abbreviation: TAZ), bathophenanthroline (abbreviation: BPhen),bathocuproine (abbreviation: BCP), or the like can be used. Thematerials described here are mainly materials having an electronmobility of greater than or equal to 10⁻⁶ cm²/Vs.

Besides the above-described materials, a high molecular compound such aspoly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)](abbreviation: PF-Py) andpoly[(9,9-dioctylfluorene-2,7-diyl)-co-(2,2′-bipyridine-6,6′-diyl)](abbreviation: PF-BPy) can be used.

Further, in this embodiment, as the donor material used for theelectron-injecting buffer 104, an alkali metal, an alkaline earth metal,a rare earth metal, a compound thereof (e.g., an alkali metal compound(e.g., an oxide such as lithium oxide, a halide, and carbonate such aslithium carbonate or cesium carbonate), an alkaline earth metal compound(e.g., an oxide, a halide, and carbonate), a rare earth metal compound(e.g., an oxide, a halide, and carbonate)), or the like can be used.Alternatively, an organic compound such as tetrathianaphthacene(abbreviation: TTN), nickelocene, or decamethylnickelocene can be used.

Note that, in this embodiment, in the first EL layer 103, theelectron-transporting layer 108 may be formed in contact with theelectron-injecting buffer 104, and that in the case where theelectron-transporting layer 108 is formed, the material having a highelectron-transporting property used for the electron-injecting buffer104 and a material having a high electron-transporting property used forthe electron-transporting layer 108 that is part of the EL layer 103 maybe the same or different.

As illustrated in FIG. 3A, the light-emitting element described in thisembodiment has a feature in that the electron-injecting buffer 104containing the material having a high electron-transporting property andthe donor material is formed between the EL layer 103 and theelectron-relay layer 105. FIG. 3B is a band diagram of this elementstructure.

In other words, the electron-injecting buffer 104 is formed, whereby theinjection barrier between the electron-relay layer 105 and the EL layer103 can be reduced; thus, electrons generated in the charge productionregion 106 can be easily injected into the first EL layer 103. Inaddition, holes generated in the charge production region 106 move tothe second EL layer 107.

Note that the structure described in this embodiment can be combinedwith any of the structures described in other embodiments asappropriate.

Embodiment 4

In Embodiment 4, as an example of the light-emitting element included inthe basic structure described in Embodiment 1, the structure of thecharge production region 106 will be described with reference to FIGS.4A and 4B.

In element structures illustrated in FIGS. 4A and 4B, the first EL layer103 and the second EL layer 107 each including a light-emitting regionare sandwiched between a pair of electrodes (the anode 101 and thecathode 102), and between the first EL layer 103 and the second EL layer107, the electron-injecting buffer 104, the electron-relay layer 105,and the charge production region 106 are stacked in this order from theanode 101 side. In FIGS. 4A and 4B, the anode 101, the cathode 102, thefirst EL layer 103, the electron-injecting buffer 104, theelectron-relay layer 105, and the second EL layer 107 can be formedusing materials similar to those described in Embodiment 1 and can havestructures similar to those described in Embodiment 1.

In the element structures illustrated in FIGS. 4A and 4B, the chargeproduction region 106 is a region that contains a material having a highhole-transporting property and an acceptor material. Note that in thecharge production region 106, electrons are drawn out from the materialhaving a high hole-transporting property by the acceptor material,whereby holes and electrons are generated.

The charge production region 106 illustrated in FIG. 4A has a structurein which a material having a high hole-transporting property and anacceptor material are contained in the same film. In that case, theacceptor material is preferably added so that the mass ratio of theacceptor material to the material having a high hole-transportingproperty is from 0.1:1 to 4.0:1, in which case carriers are easilygenerated in the charge production region 106.

In FIG. 4A, the material having a high hole-transporting property isdoped with the acceptor material, and thus an increase of the drivingvoltage can be suppressed even when the thickness of the chargeproduction region 106 is increased. Accordingly, the increase in thedriving voltage can be suppressed, and improvement of color purity byoptical adjustment can be realized. In addition, short-circuiting of thelight-emitting element can be prevented by increase of the thickness ofthe charge production region 106.

On the other hand, the charge production region 106 illustrated in FIG.4B has a structure in which a layer 106 a containing a material having ahigh hole-transporting property and a layer 106 b containing an acceptormaterial are stacked. In the charge production region 106 of thelight-emitting element illustrated in FIG. 4B, the material having ahigh hole-transporting property and the acceptor material are in contactwith each other and electrons are donated and accepted, whereby anelectron transfer complex is formed. The electron transfer complex isformed only at the interface between the layer 106 a containing thematerial having a high hole-transporting property and the layer 106 bcontaining the acceptor material. Thus, the light-emitting elementillustrated in FIG. 4B is preferable because an absorption band ofvisible light is not easily formed even when the thickness of the chargeproduction region 106 is increased.

Further, the light-emitting element illustrated in FIG. 4B is combinedwith the structure described in Embodiment 2 to make theelectron-injecting buffer 104 have a single layer of an alkali metal, analkaline earth metal, a rare earth metal, or a compound thereof, wherebythe layers between the first EL layer 103 and the second EL layer 107,that is, the electron-injecting buffer 104, the electron-relay layer105, and the charge production region 106 can be formed without dopingand the total thickness of those layers can be reduced to less than orequal to about 5 nm.

As the material having a high hole-transporting property used for thecharge production region 106, any of a variety of organic compounds suchas an aromatic amine compound, a carbazole derivative, aromatichydrocarbon, and a high molecular compound (such as an oligomer, adendrimer, and a polymer) can be used. Specifically, a material having ahole mobility of greater than or equal to 10⁻⁶ cm²/Vs is preferable.However, materials other than those can also be used as long as theyhave a hole-transporting property higher than an electron-transportingproperty.

As specific examples of the aromatic amine compound, the following canbe given: 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation:NPB or α-NPD),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), 4,4′,4″-tris(carbazol-9-yl)triphenylamine(abbreviation: TCTA) 4,4′,4″-tris(N,N-diphenylamino)triphenylamine(abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA),N,N′-bis(4-methylphenyl)-N,N′-diphenyl-p-phenylenediamine (abbreviation:DTDPPA), 4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl(abbreviation: DPAB), 4,4′-bis[N-(3-methylphenyl)-N-phenylamino]biphenyl(abbreviation: DNTPD),1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene(abbreviation: DPA3B), and the like.

As specific examples of the carbazole derivative, the following can begiven: 3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1),3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2),3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1), and the like. Besides, the following can begiven: 4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP),1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB),9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA),1,4-bis[—(N-carbazolyl)phenyl]-2,3,5,6-tetraphenylbenzene, and the like.

As specific examples of the aromatic hydrocarbon, the following can begiven: 2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation:t-BuDNA), 2-tert-butyl-9,10-di(1-naphthyl)anthracene,9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA),2-tert-butyl-9,10-bis(4-phenylphenyl)anthracene (abbreviation: t-BuDBA),9,10-di(2-naphthyl)anthracene (abbreviation: DNA),9,10-diphenylanthracene (abbreviation: DPAnth), 2-tert-butylanthracene(abbreviation: t-BuAnth), 9,10-bis(4-methyl-1-naphthyl)anthracene(abbreviation: DMNA),9,10-bis[2-(1-naphthyl)phenyl]-2-tert-butylanthracene,9,10-bis[2-(1-naphthyl)phenyl]anthracene,2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene,2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene, 9,9′-bianthryl,10,10′-diphenyl-9,9′-bianthryl,10,10′-bis(2-phenylphenyl)-9,9′-bianthryl,10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl, anthracene;tetracene; rubrene, perylene; 2,5,8,11-tetra(tert-butyl)perylene; andthe like. In addition to those, pentacene, coronene, or the like canalso be used. In this way, the aromatic hydrocarbon having a holemobility of greater than or equal to 1×10⁻⁶ cm²/Vs and 14 to 42 carbonatoms is more preferably used.

Further, the aromatic hydrocarbon may have a vinyl skeleton. As thearomatic hydrocarbon having a vinyl group, for example,4,4′-bis(2,2-diphenylvinyl)biphenyl (abbreviation: DPVBi),9,10-bis[4-(2,2-diphenylvinyl)phenyl]anthracene (abbreviation: DPVPA),and the like can be given.

Moreover, a high molecular compound such as poly(N-vinylcarbazole)(abbreviation: PVK) or poly(4-vinyltriphenylamine) (abbreviation: PVTPA)can also be used.

As the acceptor material used for the charge production region 106,7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:F₄-TCNQ), chloranil, and the like can be given. In addition, atransition metal oxide can be given. Moreover, an oxide of a metalbelonging to any of Groups 4 to 8 of the periodic table can be given.Specifically, vanadium oxide, niobium oxide, tantalum oxide, chromiumoxide, molybdenum oxide, tungsten oxide, manganese oxide, and rheniumoxide are preferable because of their high electron-transportingproperties.

Note that the structure described in this embodiment can be combinedwith any of the structures described in other embodiments asappropriate.

Embodiment 5

In Embodiment 5, another example of the light-emitting element includedin the basic structure described in Embodiment 1 will be described withreference to FIGS. 24A and 24B.

As illustrated in FIG. 24A, a light-emitting element described in thisembodiment has a structure in which the first EL layer 103 and thesecond EL layer 107 each including a light-emitting region aresandwiched between a pair of electrodes (the anode 101 and the cathode102), and between the first EL layer 103 and the second EL layer 107,the electron-injecting buffer 104, the electron-relay layer 105, and thecharge production region 106 are stacked in this order from the anode101 side.

The anode 101, the cathode 102, the electron-injecting buffer 104, theelectron-relay layer 105, and the charge production region 106 in thisembodiment can be formed using materials similar to those described inEmbodiment 1.

In this embodiment, the first EL layer 103 includes a firstlight-emitting layer 103-1 which exhibits an emission spectrum having apeak in the blue to blue-green wavelength range and a secondlight-emitting layer 103-2 which exhibits an emission spectrum having apeak in the yellow to orange wavelength range. Further, the second ELlayer 107 includes a third light-emitting layer 107-1 which exhibits anemission spectrum having a peak in the blue-green to green wavelengthrange and a fourth light-emitting layer 107-2 which exhibits an emissionspectrum having a peak in the orange to red wavelength range. Note thatthe first light-emitting layer 103-1 and the second light-emitting layer103-2 may be stacked in reverse order. Note also that the thirdlight-emitting layer 107-1 and the fourth light-emitting layer 107-2 maybe stacked in reverse order.

When the anode 101 side is positively biased and the cathode 102 side isnegatively biased in such a light-emitting element, holes injected fromthe anode 101 and electrons generated in the charge production region106 and injected through the electron-relay layer 105 and theelectron-injecting buffer 104 are recombined in the first light-emittinglayer 103-1 or the second light-emitting layer 103-2, whereby firstlight emission 330 is obtained. Furthermore, electrons injected from thecathode 102 and holes generated in the charge production region 106 arerecombined in the third light-emitting layer 107-1 or the fourthlight-emitting layer 107-2, whereby second light emission 340 isobtained.

The first light emission 330 is a combination of light emission fromboth the first light-emitting layer 103-1 and the second light-emittinglayer 103-2; thus, as shown in FIG. 24B, the first light emission 330exhibits an emission spectrum having peaks in both the blue toblue-green wavelength range and the yellow to orange wavelength range.In other words, the first EL layer 103 exhibits light emission of atwo-wavelength-type white color or a color close to white. Further, thesecond light emission 340 is a combination of light emission from boththe third light-emitting layer 107-1 and the fourth light-emitting layer107-2; thus, as shown in FIG. 24B, the second light emission 340exhibits an emission spectrum having peaks in both the blue-green togreen wavelength range and the orange to red wavelength range. In otherwords, the second EL layer 107 exhibits light emission oftwo-wavelength-type white color or a color close to white, which isdifferent from the light emission of the first EL layer 103.

Accordingly, light emission which covers the blue to blue-greenwavelength range, the blue-green to green wavelength range, the yellowto orange wavelength range, and the orange to red wavelength range isobtained by the light-emitting element in this embodiment, as a resultof combining the first light emission 330 and the second light emission340.

In this embodiment, even if, for example, the emission luminance of thefirst light-emitting layer 103-1 (which exhibits an emission spectrumhaving a peak in the blue to blue-green wavelength range) deterioratesover time or changes due to current density, deviation of chromaticityis relatively small because the contribution of the first light-emittinglayer 103-1 with respect to the entire spectrum is approximately onequarter.

Note that, although the example has been described in which the first ELlayer 103 exhibits the spectrum having peaks in both the blue toblue-green wavelength range and the yellow to orange wavelength range,and the second EL layer 107 exhibits the spectrum having peaks in boththe blue-green to green wavelength range and the orange to redwavelength range, the first EL layer 103 and the second EL layer 107each may exhibit the opposite spectrum. In other words, a structure maybe employed in which the second EL layer 107 exhibits the spectrumhaving peaks in both the blue to blue-green wavelength range and theyellow to orange wavelength range, and the first EL layer 103 exhibitsthe spectrum having peaks in both the blue-green to green wavelengthrange and the orange to red wavelength range. In addition, each of thefirst EL layer 103 and the second EL layer 107 may have a structure inwhich layers other than the light-emitting layer are stacked.

Next, materials that can be used as a light-emitting organic compoundfor the EL layer of the light-emitting element described in thisembodiment will be described. However, materials that can be applied tothe light-emitting element described in this embodiment are not limitedto those given below.

Blue to blue-green light emission can be obtained, for example, by usingperylene, 2,5,8,11-tetra-t-butylperylene (abbreviation: TBP),9,10-diphenylanthracene, or the like as a guest material, and dispersingthe guest material in a suitable host material. Alternatively, the blueto blue-green light emission can be obtained from a styrylarylenederivative such as 4,4′-bis(2,2-diphenylvinyl)biphenyl (abbreviation:DPVBi), or an anthracene derivative such as 9,10-di-2-naphthylanthracene(abbreviation: DNA) or 9,10-bis(2-naphthyl)-2-t-butylanthracene(abbreviation: t-BuDNA). Further alternatively, a polymer such aspoly(9,9-dioctylfluolene) may be used. Further, as a guest material forblue light emission, a styrylamine derivative is preferable. As examplesof the styrylamine derivative,N,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine(abbreviation: YGA2S),N,N′-diphenyl-N,N′-bis(9-phenyl-9H-carbazol-3-yl)stilbene-4,4′-diamine(abbreviation: PCA2S), and the like can be given. In particular, YGA2Sis preferable because it has a peak at around 450 nm. Further, as a hostmaterial, an anthracene derivative is preferable;9,10-bis(2-naphthyl)-2-t-butylanthracene (abbreviation: t-BuDNA) and9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA) aresuitable. In particular, CzPA is preferable because of itselectrochemical stability.

Blue-green to green light emission can be obtained, for example, byusing a coumarin dye such as coumarin 30 or coumarin 6;bis[2-(2,4-difluorophenyl)pyridinato]picolinatoiridium (abbreviation:FIrpic); bis(2-phenylpyridinato)acetylacetonatoiridium (abbreviation:Ir(ppy)₂(acac)); or the like as a guest material and dispersing theguest material in a suitable host material. Alternatively, theblue-green to green light emission can be obtained by dispersingperylene or TBP given above in an appropriate host material at a highconcentration of greater than or equal to 5 wt %. Further alternatively,the blue-green to green light emission can be obtained from a metalcomplex such as BAlq, Zn(BTZ)₂, orbis(2-methyl-8-quinolinolato)chlorogallium (Ga(mq)₂Cl). Furtheralternatively, a polymer such as poly (p-phenylenevinylene) may be used.Further, an anthracene derivative is preferably used as a guest materialof a blue-green to green light-emitting layer, in which case highemission efficiency can be obtained. For example, when9,10-bis{4-[N-(4-diphenylamino)phenyl-N-phenyl]aminophenyl}-2-tert-butylanthracene(abbreviation: DPABPA) is used, highly efficient blue-green lightemission can be obtained. Further, an anthracene derivative in which anamino group has been substituted into the 2-position is preferably used,in which case highly efficient green light emission can be obtained. Inparticular, N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA) is suitable because of its long life. As a hostmaterial for those materials, an anthracene derivative is preferable;CzPA, which is given above, is preferable because of its electrochemicalstability. Further, in the case of manufacturing a light-emittingelement in which green light emission and blue light emission arecombined and which has two peaks in the blue to green wavelength range,an anthracene derivative having an electron-transporting property, suchas CzPA is preferably used as a host material for a blue light-emittinglayer and an aromatic amine compound having a hole-transportingproperty, such as NPB is preferably used as a host material for a greenlight-emitting layer, in which case light emission can be obtained at aninterface between the blue light-emitting layer and the greenlight-emitting layer. In other words, in such a case, an aromatic aminecompound like NPB is preferable as a host material for a greenlight-emitting material such as 2PCAPA.

Yellow to orange light emission can be obtained, for example, by usingrubrene,4-(dicyanomethylene)-2-[p-(dimethylamino)styryl]-6-methyl-4H-pyran(abbreviation: DCM1),4-(dicyanomethylene)-2-methyl-6-(9-julolidyl)ethynyl-4H-pyran(abbreviation: DCM2), bis[2-(2-thienyl)pyridinato]acetylacetonatoiridium(abbreviation: Ir(thp)₂(acac)),bis(2-phenylquinolinato)acetylacetonatoiridium (abbreviation:Ir(pq)₂(acac)), or the like as a guest material and dispersing the guestmaterial in a suitable host material. In particular, a tetracenederivative such as rubrene is preferable as a guest material because ofits high efficiency and chemical stability. As a host material in thatcase, an aromatic amine compound such as NPB is preferable.Alternatively, a metal complex such as bis(8-quinolinolato)zinc(abbreviation: Znq₂), bis[2-cinnamoyl-8-quinolinolato]zinc(abbreviation: Znsq₂), or the like can be used as a host material.Further alternatively, a polymer such aspoly(2,5-dialkoxy-1,4-phenylenevinylene) may be used.

Orange to red light emission can be obtained, for example, using 4-(dicyanomethylene)-2,6-bis[p-(dimethylamino)styryl]-4H-pyran (abbreviation:BisDCM),2-(2-{2-[4-(dimethylamino)phenyl]ethenyl}-6-methyl-4H-pyran-4-ylidene)propanedinitrile(abbreviation: DCM1),4-(dicyanomethylene)-2-methyl-6-(9-julolidyl)ethynyl-4H-pyran(abbreviation: DCM2), bis[2-(2-thienyl)pyridinato]acetylacetonatoiridium(abbreviation: Ir(thp)₂(acac)), or the like as a guest material anddispersing the guest material in a suitable host material.Alternatively, the orange to red light emission can be obtained from ametal complex such as bis(8-quinolinolato)zinc (abbreviation: Znq₂) orbis[2-cinnamoyl-8-quinolinolato) zinc (abbreviation: Znsq₂). Furtheralternatively, a polymer such as poly(3-alkylthiophene) may be used. Asa guest material which exhibits red light emission, a 4H-pyranderivative such as4-(dicyanomethylene)-2,6-bis[p-(dimethylamino)styryl]-4H-pyran(abbreviation: BisDCM),2-(2-{2-[4-(dimethylamino)phenyl]ethenyl}-6-methyl-4H-pyran-4-ylidene)propanedinitrile(abbreviation: DCM1),4-(dicyanomethylene)-2-methyl-6-(9-julolidyl)ethynyl-4H-pyran(abbreviation: DCM2),{2-isopropyl-6-[2-(2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCJTI), or{2,6-bis[2-(2,3,6,7-tetrahydro-8-methoxy-1,1,7,7-tetramethyl-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: BisDCJTM) is preferable because of its high efficiency.In particular, DCJTI and BisDCJTM are preferable because they have alight emission peak at around 620 nm.

As the appropriate host material in the above-described structures, ahost material which has a shorter wavelength than the light-emittingorganic compound or a host material which has a large energy gap ispreferably used. Specifically, a hole-transporting material or anelectron-transporting material typified by the examples given inEmbodiment 1 can be selected as appropriate. Alternatively,4,4′-bis(N-carbazolyl)-biphenyl (abbreviation: CBP),4,4′,4″-tris(N-carbazolyl)triphenylamine (abbreviation: TCTA),1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB), or thelike may be used.

White light emission which covers the blue to blue-green wavelengthrange, the blue-green to green wavelength range, the yellow to orangewavelength range, and the orange to red wavelength range is obtained bythe light-emitting element described in this embodiment, as a result ofcombining the emission spectrum of the first EL layer and the emissionspectrum of the second EL layer.

Note that light may be made more like natural light having a continuousemission spectrum in such a manner that the thickness of each stackedlayer is adjusted and slight interference of light is deliberatelycaused so that generation of a projected sharp peak is suppressed and atrapezoidal emission spectrum is obtained. In addition, the position ofa peak of an emission spectrum can also be changed by adjusting thethickness of each stacked layer and intentionally causing slightinterference of light. By adjusting the thickness of each stacked layerso that a plurality of peak intensities which appear in an emissionspectrum are made roughly the same and by decreasing the intervalsbetween the peaks, white light emission having an emission spectrumwhich is closer to a trapezoidal shape can be obtained.

Note that in this embodiment, the EL layer is described in which in eachof the plurality of light-emitting layers, emission colors that arecomplementary colors are combined to obtain white light emission.Hereinafter, a specific structure of an EL layer which exhibits whitelight emission by the relationship of complementary colors will bedescribed.

The EL layer provided in the light-emitting element described in thisembodiment has a structure in which, for example, a first layercontaining a material having a high hole-transporting property and afirst light-emitting material; a second layer containing a materialhaving a high hole-transporting property and a second light-emittingmaterial; and a third layer containing a material having a highelectron-transporting property and the second light-emitting materialare stacked in this order from the anode 101 side.

Both the first light-emitting material and the second light-emittingmaterial should emit light in order that white light emission isobtained in the EL layers of the light-emitting element described inthis embodiment. Thus, in order to adjust the transporting properties ofcarriers in the EL layers, both the material having a highhole-transporting property and the material having a highelectron-transporting property are preferably used as host materials.Note that as the material having a high hole-transporting property orthe material having a high electron-transporting property which can beused for the EL layers, the materials given as examples in Embodiment 1can be used as appropriate.

Further, as the first light-emitting material and the secondlight-emitting material, materials emitting light of colors that arecomplementary colors can be selected. As the complementary colors, colorcombinations of blue and yellow, blue-green and red, and the like can begiven. A material which emits blue, yellow, blue-green, or red light maybe selected as appropriate from, for example, the light-emittingmaterials given above. Note that the emission wavelength of the firstlight-emitting material is made to be shorter than the emissionwavelength of the second light-emitting material, whereby part ofexcitation energy of the second light-emitting material is transferredto the first light-emitting material, so that the first light-emittingmaterial can be made to emit light. Thus, in the light-emitting elementof this embodiment, the emission peak wavelength of the secondlight-emitting material is preferably shorter than the emission peakwavelength of the first light-emitting material.

In the structure of the light-emitting element described in thisembodiment, both light emission from the first light-emitting materialand light emission from the second light-emitting material can beobtained, and the emission color of the first light-emitting materialand the emission color of the second light-emitting material arecomplementary colors, and accordingly white light emission can beobtained. In addition, the structure of the light-emitting elementdescribed in this embodiment is employed, whereby a light-emittingelement with a long life can be obtained.

Note that the structure described in this embodiment can be combinedwith any of the structures described in other embodiments asappropriate.

Embodiment 6

In Embodiment 6, one mode of a light-emitting device including thelight-emitting element described in any of the above embodiments will bedescribed with reference to FIGS. 5A to 5C. FIGS. 5A to 5C arecross-sectional views of light-emitting devices.

In each of FIGS. 5A to 5C, a portion surrounded by a rectangle of dottedlines corresponds to a transistor 11 which is provided for driving alight-emitting element 12. The light-emitting element 12 includes alayer 15 containing an organic compound between a first electrode 13 anda second electrode 14. The layer containing an organic compound includesn (n is a natural number of two or more) EL layers, where between m-th(m is a natural number, 1≦m≦n−1) EL layer and (m+1)-th EL layer, anelectron-injecting buffer, an electron-relay layer, and a chargeproduction region are provided in this order from an anode side.Further, in each of the EL layers, at least a light-emitting layer isprovided, and a hole-injecting layer, a hole-transporting layer, anelectron-transporting layer, or an electron-injecting layer is providedas appropriate in addition to the light-emitting layer. In other words,the light-emitting element 12 has a structure like the one described inany of Embodiments 1 to 4. A drain region of the transistor 11 iselectrically connected to the first electrode 13 by a wiring 17penetrating a first interlayer insulating film 16 (16 a, 16 b, and 16c). The light-emitting element 12 is separated from otheradjacently-provided light-emitting elements by partition layers 18. Thelight-emitting device of this embodiment having such a structure isprovided over a substrate 10 in this embodiment.

The transistor 11 illustrated in each of FIGS. 5A to 5C is a top-gatetype transistor in which a gate electrode is provided on an oppositeside to the substrate with a semiconductor layer interposed between thesubstrate and the gate electrode. However, there is no particularlimitation on the structure of the transistor 11; for example, thetransistor 11 may be of bottom-gate type. In the case where thetransistor 11 is of bottom-gate type, the transistor 11 may have astructure in which a protective film is formed over the semiconductorlayer used to form a channel (a channel protective type) or a structurein which part of the semiconductor layer used to form a channel has adepression (a channel etch type).

Further, the semiconductor layer included in the transistor 11 may beeither crystalline or non-crystalline. Alternatively, a microcrystallinesemiconductor, an oxide semiconductor, or the like may be used.

For the oxide semiconductor layer, composite oxide of an elementselected from indium, gallium, aluminum, zinc, and tin can be used. Forexample, zinc oxide (ZnO), indium oxide containing zinc oxide (IZO), andoxide containing indium oxide, gallium oxide, and zinc oxide (IGZO) canbe given. As a specific example of the crystalline semiconductor layer,a layer formed of single crystal or polycrystalline silicon, silicongermanium, or the like can be given. It may be formed by lasercrystallization or may be formed by crystallization through a solidphase growth method using, for example, nickel.

In the case where the semiconductor layer is formed using an amorphousmaterial, for example, amorphous silicon, it is preferable that thelight-emitting device have a circuit in which the transistor 11 andother transistors (transistors constituting a circuit for driving thelight-emitting element) are all n-channel transistors. Further, manyoxide semiconductors, for example, zinc oxide (ZnO), indium oxidecontaining zinc oxide (IZO), oxide containing indium oxide, galliumoxide, and zinc oxide (IGZO), are n-type semiconductors; thus, atransistor in which any of those compounds is contained in an activelayer is an n-channel transistor. In a case other than the above, alight-emitting device may have a circuit including either an n-channeltransistor or a p-channel transistor, or may have a circuit includingboth an n-channel transistor and a p-channel transistor.

Further, the first interlayer insulating film 16 may be a multilayer asillustrated in FIGS. 5A and 5C, or may be a single layer. Note that theinterlayer insulating film 16 a is formed of an inorganic material suchas silicon oxide or silicon nitride; the interlayer insulating film 16 bis formed of acrylic, siloxane (an organic group including a skeleton ofa silicon-oxygen bond (Si—O bond) and containing at least hydrogen as asubstituent) or a self-planarizing material which can be formed as afilm by an application method, such as silicon oxide. In addition, theinterlayer insulating film 16 c is formed of a silicon nitride filmcontaining argon (Ar). Note that there is no particular limitation onthe material forming each layer, and a material other than the abovematerials may also be used. A layer formed using a material other thanthe above materials may be further combined. As described above, thefirst interlayer insulating films 16 a to 16 c may be formed usingeither an inorganic material or an organic material, or both of them.

As for the partition layer 18, the radius of curvature of the edgeportion preferably changes continuously. In addition, the partitionlayer 18 is formed using acrylic, siloxane, resist, silicon oxide, orthe like. Note that the partition layer 18 may be formed using either aninorganic material or an organic material, or both of them.

Note that, although the structure in which only the first interlayerinsulating films 16 a to 16 c are provided between the transistor 11 andthe light-emitting element 12 is illustrated in each of FIGS. 5A and 5C,the structure illustrated in FIG. 5B may be employed in which a secondinterlayer insulating film 19 (19 a and 19 b) is provided in addition tothe first interlayer insulating film 16 (16 a and 16 b). In thelight-emitting device illustrated in FIG. 5B, the first electrode 13penetrates the second interlayer insulating film 19 to be connected tothe wiring 17.

The second interlayer insulating film 19 may be a multilayer like thefirst interlayer insulating film 16 or may be a single layer. The secondinterlayer insulating film 19 a is formed of acrylic, siloxane (anorganic group including a skeleton of a silicon-oxygen bond (Si—O bond)and containing at least hydrogen as a substituent), or aself-planarizing substance which can be formed as a film by anapplication method, such as silicon oxide. The second interlayerinsulating film 19 b is formed of a silicon nitride film containingargon (Ar). Note that there is no particular limitation on the materialforming each layer, and a material other than the above materials mayalso be used. A layer formed of a material other than the abovematerials may be further combined. As described above, the secondinterlayer insulating film 19 may be formed using either an inorganicmaterial or an organic material, or both of them.

In the case where both the first electrode and the second electrode inthe light-emitting element 12 are formed using a light-transmittingmaterial, emitted light can be extracted from both the first electrode13 and the second electrode 14 as indicated by the outline arrows inFIG. 5A. In addition, in the case where only the second electrode 14 isformed using a light-transmitting material, emitted light can beextracted from only the second electrode 14 as indicated by the outlinearrow in FIG. 5B. In that case, the first electrode 13 is preferablyformed using a material having high reflectivity, or a film formed usinga material having high reflectivity (reflective film) is preferablyprovided under the first electrode 13. Furthermore, in the case whereonly the first electrode 13 is formed using a light-transmittingmaterial, emitted light can be extracted from only the first electrode13 as indicated by the outline arrow in FIG. 5C. In that case, thesecond electrode 14 is preferably formed using a material having highreflectivity, or a reflective film is preferably formed above the secondelectrode 14.

Further, in the light-emitting element 12, the layer 15 may be stackedso that the light-emitting element 12 is driven when a voltage isapplied so that the potential of the second electrode 14 becomes higherthan that of the first electrode 13, or the layer 15 may be stacked sothat the light-emitting element 12 is driven when a voltage is appliedso that the potential of the second electrode 14 becomes lower than thatof the first electrode 13. In the former case, the transistor 11 is ann-channel transistor, while in the latter case, the transistor 11 is ap-channel transistor.

Note that, although only one light-emitting element is illustrated ineach of the cross-sectional views of FIGS. 5A to 5C, a plurality oflight-emitting elements are arranged in matrix in a pixel portion.Further, in the case where color display of color components, R (red), G(green), and B (blue), is performed, a plurality of light-emittingelements which provide three kinds of light emissions (R, G, and B) areformed in the pixel portion. In addition, the color components are notlimited to three colors, and color components of four colors or more maybe used or a color other than R, and B may be used. For example, whitemay be added so that R, G, B, and W (W means white) can be used.

As a manufacturing method of light-emitting elements of different colorcomponents, the following method can be used: a method in which ELlayers of different colors are separately arranged; a method in whichall EL layers are formed so as to emit white light and the EL layers arecombined with color filters, whereby light-emitting elements ofdifferent color components are obtained; a method in which all EL layersare formed so as to emit blue light or light with a shorter wavelengththan blue light and the EL layers are combined with color conversionlayers, whereby light-emitting elements of different color componentsare obtained; or the like.

As described above, in this embodiment, an active matrix light-emittingdevice in which the driving of the light-emitting element is controlledby the transistor is described. However, a passive matrix light-emittingdevice in which a light-emitting element is driven without providing anelement for driving, such as a transistor, over the same substrate asthe light-emitting element may be employed. FIG. 6A is a perspectiveview of a passive matrix light-emitting device manufactured byapplication of the light-emitting element described in any ofEmbodiments 1 to 4. In addition, FIG. 6B is a cross-sectional view takenalong a dashed line X-Y of FIG. 6A.

In FIGS. 6A and 6B, over a substrate 951, a layer 955 containing anorganic compound is provided between an electrode 952 and an electrode956. The layer containing an organic compound includes n (n is a naturalnumber of two or more) EL layers, where between m-th (m is a naturalnumber, 1≦m≦n−1) EL layer and (m+1)-th EL layer, an electron-injectingbuffer, an electron-relay layer, and a charge production region areprovided in this order from an anode side. Further, in each of the ELlayers, at least a light-emitting layer is provided, and ahole-injecting layer, a hole-transporting layer, anelectron-transporting layer, or an electron-injecting layer is providedas appropriate in addition to the light-emitting layer. End portions ofthe electrode 952 are covered with an insulating layer 953. Then, apartition layer 954 is provided over the insulating layer 953. Thepartition layer 954 preferably has tapered sidewalls with such a slopethat the distance between opposite sidewalls decreases toward thesubstrate surface. In other words, a cross section of the partitionlayer 954 in the direction of a narrow side is trapezoidal, and a base(a side facing in a similar direction to a plane direction of theinsulating layer 953 and being in contact with the insulating layer 953)is shorter than an upper side (a side facing in a similar direction tothe plane direction of the insulating layer 953 and not being in contactwith the insulating layer 953). The partition layer 954 is provided inthis manner, whereby a defect of the light-emitting element due tostatic electricity or the like can be prevented. The passive matrixlight-emitting device can also be driven with low power consumption whenit includes the light-emitting element described in any of Embodiments 1to 4.

The light-emitting element described as an example in any of the aboveembodiments is used in the light-emitting device described in thisembodiment; thus, the light-emitting device can have high luminance, canbe driven at low voltage, and consumes less power.

Embodiment 7

In Embodiment 7, electronic devices each of which includes, as partthereof, the light-emitting device described in Embodiment 6 will bedescribed. Electronic devices described in Embodiment 7 each include adisplay portion which includes the light-emitting element described inany of Embodiments 1 to 4, has high luminance, is driven at low voltage,and consumes less power.

As examples of the electronic devices of this embodiment, the followingcan be given: cameras such as video cameras and digital cameras, goggletype displays, navigation systems, audio replay devices (e.g., car audiosystems and audio systems), computers, game machines, portableinformation terminals (e.g., mobile computers, cellular phones, portablegame machines, and electronic book readers), image replay devices inwhich a recording medium is provided (specifically, devices that arecapable of replaying recording media such as digital versatile discs(DVDs) and equipped with a display device that can display an image),and the like. Specific examples of those electronic devices areillustrated in FIGS. 7A to 7E.

FIG. 7A illustrates an example of a portable information terminal device9200. The portable information terminal device 9200 incorporates acomputer and therefore can process a variety of types of data. As anexample of the portable information terminal device 9200, a personaldigital assistant (PDA) can be given.

The portable information terminal device 9200 has two housings: ahousing 9201 and a housing 9203. The housing 9201 and the housing 9203are joined with a joining portion 9207 such that the portableinformation terminal device 9200 can be foldable. A display portion 9202is incorporated in the housing 9201, and the housing 9203 is providedwith a keyboard 9205. Needless to say, the structure of the portableinformation terminal device 9200 is not limited to the one describedabove, and the portable information terminal device 9200 may be providedwith other accessories as appropriate. In the display portion 9202,light-emitting elements similar to those described in any of the aboveembodiments are arranged in matrix. The light-emitting elements havefeatures of high luminance, low driving voltage, and low powerconsumption. The display portion 9202 including those light-emittingelements also has similar features; thus, low power consumption of thisportable information terminal device can be achieved.

FIG. 7B illustrates an example of a digital video camera 9500 accordingto this embodiment. The digital video camera 9500 includes a displayportion 9503 incorporated in a housing 9501 and various operationportions. Note that the structure of the digital video camera 9500 isnot particularly limited and the digital video camera 9500 may beprovided with other accessories as appropriate.

In this digital video camera, the display portion 9503 includeslight-emitting elements similar to those described in any of the aboveembodiments, which are arranged in matrix. The light-emitting elementshave features of low driving voltage, high luminance, and low powerconsumption. The display portion 9503 including those light-emittingelements also has similar features; therefore, low power consumption ofthis digital video camera can be achieved.

FIG. 7C illustrates an example of a cellular phone 9100 according tothis embodiment. The cellular phone 9100 has two housings: a housing9102 and a housing 9101. The housing 9102 and the housing 9101 arejoined with a joining portion 9103 such that the cellular phone can befoldable. A display portion 9104 is incorporated in the housing 9102,and the housing 9101 is provided with operation keys 9106. Note that thestructure of the cellular phone 9100 is not particularly limited and thecellular phone 9100 may be provided with other accessories asappropriate.

In this cellular phone, the display portion 9104 includes light-emittingelements similar to those described in any of the above embodiments,which are arranged in matrix. The light-emitting elements have featuresof high luminance, low driving voltage, and low power consumption. Thedisplay portion 9104 including those light-emitting elements also hassimilar features; therefore, low power consumption of this cellularphone can be achieved. As a backlight of a display provided for acellular phone or the like, the light-emitting element described in anyof the above embodiments may be used.

FIG. 7D illustrates an example of a portable computer 9400. The computer9400 has two housings: a housing 9401 and a housing 9404 that are joinedsuch that the computer 9400 can be opened and closed. A display portion9402 is incorporated in the housing 9401, and the housing 9404 isprovided with a key board 9403 and the like. Note that the structure ofthe computer 9400 is not particularly limited and the computer 9400 maybe provided with other accessories as appropriate.

In this computer, the display portion 9402 includes light-emittingelements similar to those described in the any of above embodiments,which are arranged in matrix. The light-emitting elements have featuresof high luminance, low driving voltage, and low power consumption. Thedisplay portion 9402 including those light-emitting elements also hassimilar features; therefore, low power consumption of this computer canbe achieved.

FIG. 7E illustrates an example of a television set 9600. In thetelevision set 9600, a display portion 9603 is incorporated in a housing9601. The display portion 9603 can display images. Here, the housing9601 is supported by a stand 9605.

The television set 9600 can be operated with an operation switch of thehousing 9601 or a separate remote controller 9610. Channels can beselected and volume can be controlled with an operation key 9609 of theremote controller 9610, whereby images displayed on the display portion9603 can be controlled. Furthermore, the remote controller 9610 may beprovided with a display portion 9607 for displaying informationoutputted from the remote controller 9610.

Note that the television set 9600 is provided with a receiver, a modem,and the like. With the use of the receiver, a general televisionbroadcast can be received. Moreover, when the television set isconnected to a communication network with or without wires via themodem, one-way (from a sender to a receiver) or two-way (between asender and a receiver or between receivers) information communicationcan be performed.

In at least one of the display portion 9603 and the display portion 9607of this television set, light-emitting elements similar to thosedescribed in any of the above embodiments are arranged in matrix. Thelight-emitting elements have features of high luminance, low drivingvoltage, and low power consumption. The display portion including thoselight-emitting elements also has the similar features.

As described above, the application range of the light-emitting devicedescribed in the above embodiment is so wide that this light-emittingdevice can be applied to electronic devices in all fields. With use ofthe light-emitting elements described in Embodiments 1 to 4, anelectronic device having a low power consumption display portion whichexhibits high luminance light emission can be provided.

Further, the light-emitting device described in the above embodiment canalso be used as a lighting device. An embodiment in which thelight-emitting device described in the above embodiment is used as alighting device will be described with reference to FIG. 8.

FIG. 8 illustrates an example in which the light-emitting device, anexample of which is described in the above embodiment, is used as atable lamp that is a lighting device and an interior lighting device.The table lamp illustrated in FIG. 8 includes a light source 3000. Forthe light source 3000, the light-emitting device, an example of which isdescribed in the above embodiment is used. Thus, a low power consumptionlight-emitting device can be obtained. Since this light-emitting devicecan have a larger area, the light-emitting device can be used as alighting device having a large area. In addition, this light-emittingdevice is thin and consumes less power and therefore can be used as alighting device which achieves reduction in thickness and powerconsumption of the lighting device. Moreover, this light-emitting devicecan be flexible and therefore can be used as, for example, a roll-typelighting device like a lighting device 3002. As described above, thetelevision set described with reference to FIG. 7E can be installed in aroom where the light-emitting device described in this embodiment isused as the indoor lighting devices 3001 and 3002.

As described above, the application range of the light-emitting devicedescribed in Embodiment 6 is so wide that the light-emitting device canbe applied to electronic devices in all fields. Note that thisembodiment can be combined with any of Embodiments 1 to 5 asappropriate.

Example 1

In Example 1, a light-emitting element that is one embodiment of thepresent invention will be described with reference to FIGS. 9A and 9B.Chemical formulae of the materials used in this example and Examples 2to 6 are shown below.

A method for manufacturing a light-emitting element 1 and a referencelight-emitting element 1 in this example will be described below.

First, the light-emitting element 1 will be described (see FIG. 9A).Indium tin oxide containing silicon oxide was deposited over a glasssubstrate 2100 by a sputtering method to form a first electrode 2101.The first electrode 2102 has a thickness of 110 nm and an area of 2 mm×2mm.

Next, the substrate on which the first electrode 2101 was formed wasfixed to a substrate holder provided in a vacuum evaporation apparatusin such a way that a surface of the substrate on which the firstelectrode 2101 was formed faced downward, and then the pressure wasreduced to about 10⁻⁴ Pa. After that,4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) thatis a material having a high hole-transporting property andmolybdenum(VI) oxide that is an acceptor material were co-evaporated onthe first electrode 2101 to form a first charge production region 2103 acontaining a composite material of an organic compound and an inorganiccompound. The thickness of the first charge production region 2103 a was50 nm. The weight ratio of NPB to molybdenum(VI) oxide was adjusted tobe 4:1 (=NPB:molybdenum oxide). Note that the co-evaporation method isan evaporation method in which evaporation is carried out from aplurality of evaporation sources at the same time in one treatmentchamber.

Next, NPB was deposited to a thickness of 10 nm on the first chargeproduction region 2103 a by an evaporation method using resistanceheating to form a hole-transporting layer 2103 b.

Furthermore, 9-[4-(N-carbazolyl)]phenyl-10-phenylanthracene(abbreviation: CzPA) and9,10-bis{4-[N-(4-diphenylamino)phenyl-N-phenyl]aminophenyl}-2-tert-butylanthracene(abbreviation: DPABPA) were co-evaporated to form a light-emitting layer2103 c with a thickness of 30 nm on the hole-transporting layer 2103 b.Here, the weight ratio of CzPA to DPABPA was adjusted to be 1:0.1(=CzPA:DPABPA). Note that CzPA is a material having anelectron-transporting property and DPABPA that is a guest material is amaterial exhibiting blue-green light emission.

After that, tris(8-quinolinolato)aluminum (abbreviation: Alq) wasdeposited on the light-emitting layer 2103 c to a thickness of 10 nm byan evaporation method using resistance heating to form anelectron-transporting layer 2103 d. Accordingly, a first EL layer 2103including the first charge production region 2103 a, thehole-transporting layer 2103 b, the light-emitting layer 2103 c, and theelectron-transporting layer 2103 d was formed.

Next, bathophenanthroline (abbreviation: BPhen) and lithium (Li) wereco-evaporated to form an electron-buffer 2104 with a thickness of 10 nmon the electron-transporting layer 2103 d. Here, the weight ratio ofBPhen to Li was adjusted to be 1:0.02 (=BPhen:Li).

Next, 3,4,9,10-perylenetetracarboxylic-bis-benzimidazole (abbreviation:PTCBI) was evaporated to form an electron-relay layer 2105 with athickness of 3 nm on the electron-injecting buffer 2104. Note that theLUMO level of PTCBI is approximately −4.0 eV according to the result ofcyclic voltammetry (CV).

Next, NBP that is a material having a high hole-transporting propertyand molybdenum(VI) oxide that is an acceptor material were co-evaporatedon the electron-relay layer 2105 to form a second charge productionregion 2106. The thickness of the second charge production region 2106was 20 nm. The weight ratio of NPB to molybdenum(VI) oxide was adjustedto be 4:1 (=NPB:molybdenum oxide).

Next, a second EL layer 2107 was formed on the second charge productionregion 2106. A method for manufacturing the second EL layer 2107 will bedescribed below. First, NPB was deposited to a thickness of 10 nm on thesecond charge production region 2106 to form a hole-transporting layer2107 a by an evaporation method using resistance heating.

After that, tris(8-quinolinolato)aluminum (abbreviation: Alq) and4-dicyanomethylene-2-isopropyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolidin-9-yl)ethenyl]-4H-pyran(abbreviation: DCJTI) were co-evaporated to form a light-emitting layer2107 b with a thickness of 40 nm on the hole-transporting layer 2107 a.Here, the weight ratio of Alq to DCJTI was adjusted to be 1:0.01(=Alq:DCJTI). Note that Alq is a material having anelectron-transporting property and DCJTI that is a guest material is amaterial exhibiting red light emission.

Next, Alq with a thickness of 10 nm and BPhen with a thickness of 20 nmwere stacked on the light-emitting layer 2107 b by evaporation to forman electron-transporting layer 2107 c. Then, lithium fluoride (LiF) wasevaporated to a thickness of 1 nm on the electron-transporting layer2107 c to form an electron-injecting layer 2107 d. Accordingly, thesecond EL layer 2107 including the hole-transporting layer 2107 a, thelight-emitting layer 2107 b, the electron-transporting layer 2107 c, andthe electron-injecting layer 2107 d was formed.

Lastly, aluminum was deposited to a thickness of 200 nm on theelectron-injecting layer 2107 d by an evaporation method usingresistance heating to form a second electrode 2102. Accordingly, thelight-emitting element 1 was manufactured.

Next, the reference light-emitting element 1 will be described (see FIG.9B). The reference light-emitting element 1 has the structure of thelight-emitting element 1, from which the electron-relay layer 2105 isremoved. The other layers were formed by manufacturing methods similarto those of the light-emitting element 1. As for the referencelight-emitting element 1, after the electron-injecting buffer 2104 wasformed, the second charge production region 2106 was formed on theelectron-injecting buffer 2104. Accordingly, the referencelight-emitting element 1 of this example was obtained.

Table 1 below shows the element structures of the light-emitting element1 and the reference light-emitting element 1.

TABLE 1 2103 2101 2103a 2103b 2103c 2103d 2104 2105 2106 Light- ITSONPB: NPB CzPA: Alq BPhen:Li PTCBI NPB: emitting 110 nm MoOx 10 nm DPABPA10 nm (=1:0.02) 3 nm MoOx element 1 (=4:1) (=1:0.1) 10 nm (=4:1) 50 nm30 nm 20 nm Reference ITSO NPB: NPB CzPA: Alq BPhen:Li — NPB: light- 110nm MoOx 10 nm DPABPA 10 nm (=1:0.02) MoOx emitting (=4:1) (=1:0.1) 10 nm(=4:1) element 1 50 nm 30 nm 20 nm 2107 2107a 2107b 2107c 2107d 2102Light- NPB Alq:DCJTI Alq BPhen LiF Al emitting 10 mn (=1:0.01) 10 nm 20nm 1 nm 200 nm element 1 40 nm Reference NPB Alq:DCJTI Alq BPhen LiF Allight- 10 nm (=1:0.01) 10 nm 20 nm 1 nm 200 nm emitting 40 nm element 1

The thus obtained light-emitting element 1 and reference light-emittingelement 1 were sealed in a glove box under a nitrogen atmosphere so thatthey were not exposed to atmospheric air. After that, the operatingcharacteristics of these light-emitting elements were measured. Themeasurement was carried out at room temperature (under an atmosphere inwhich the temperature was kept at 25° C.).

FIG. 10 shows voltage-luminance characteristics of the light-emittingelement 1 and the reference light-emitting element 1. In FIG. 10, thehorizontal axis represents applied voltage (V) and the vertical axisrepresents luminance (cd/m²). In addition, FIG. 11 shows currentdensity-luminance characteristics. In FIG. 11, the horizontal axisrepresents voltage (V) and the vertical axis represents current density(mA/cm²). Moreover, Table 2 below shows initial values of maincharacteristics of the light-emitting element 1 and the referencelight-emitting element 1 at around 1000 cd/m².

TABLE 2 Voltage Chromaticity Current efficiency (V) (x, y) (cd/A)Light-emitting 11 (0.31, 0.28) 3.7 element 1 Reference 12 (0.32, 0.29)4.2 light-emitting element 1

Note that, as seen from CIE chromaticity coordinates of Table 2, thelight-emitting element 1 and the reference light-emitting element 1 bothexhibit white light emission. That is because blue-green light emissionderived from DPABPA contained in the first EL layer 2103 and red lightemission derived from DCJTI contained in the second EL layer 2107 wereboth obtained.

According to FIG. 10, the light-emitting element 1 in which theelectron-relay layer is provided can have higher luminance than thereference light-emitting element 1 when the same voltage is applied tothese light-emitting elements. In addition, according to FIG. 11, thelight-emitting element 1 has a higher current density than the referencelight-emitting element 1.

Accordingly, it was confirmed that the light-emitting element 1 of thisexample had characteristics as a light-emitting element and functionedwell. In addition, it was confirmed that the light-emitting element 1was a light-emitting element capable of being driven at low voltage.

Example 2

In Example 2, a light-emitting element that is one embodiment of thepresent invention will be described with reference to FIGS. 12A and 12B.Note that as for a light-emitting element and a reference light-emittingelement described in this example, the same or similar parts as/to orparts having the same or similar functions as/to those described inExample 1 are denoted by the same reference numerals as those of Example1, and the description of them will not be repeated.

Hereinafter, a method for manufacturing a light-emitting element 2 and areference light-emitting element 2 of this example will be described.

First, the light-emitting element 2 will be described (see FIG. 12A).The light-emitting element 2 of this example was manufactured in amanner similar to that of the light-emitting element 1 described inExample 1 up to the formation of the electron-relay layer 2105. In thelight-emitting element 2 of this example, molybdenum(VI) oxide that isan acceptor material with a thickness of 20 nm and NPB that is amaterial having a high hole-transporting property with a thickness of 10nm were stacked on the electron-relay layer 2105 by evaporation to formthe second charge production region 2106.

Next, a second EL layer 2108 was formed on the second charge productionregion 2106. A method for manufacturing the second EL layer 2108 will bedescribed below. First, Alq and DCJTI were co-evaporated to form alight-emitting layer 2108 a with a thickness of 40 nm on the secondcharge production region 2106. Here, the weight ratio of Alq to DCJTIwas adjusted to be 1:0.01 (=Alq:DCJTI). Note that Alq is a materialhaving an electron-transporting property and DCJTI that is a guestmaterial is a material exhibiting red light emission.

Next, Alq with a thickness of 10 nm and BPhen with a thickness of 20 nmwere stacked on the light-emitting layer 2108 a by evaporation to forman electron-transporting layer 2108 b. Then, lithium fluoride (LiF) wasevaporated to a thickness of 1 nm on the electron-transporting layer2108 b to form an electron-injecting layer 2108 c. Accordingly, thesecond EL layer 2108 including the light-emitting layer 2108 a, theelectron-transporting layer 2108 b, and the electron-injecting layer2108 c was formed.

Lastly, aluminum was deposited on the electron-injecting layer 2108 c toa thickness of 200 nm by an evaporation method using resistance heatingto form the second electrode 2102. Accordingly, the light-emittingelement 2 was manufactured.

Next, the reference light-emitting element 2 will be described (see FIG.12B). The reference light-emitting element 2 of this example has thestructure of the light-emitting element 2, from which the electron-relaylayer 2105 is removed. The other layers were formed by manufacturingmethods similar to those of the light-emitting element 2. As for thereference light-emitting element 2, after the electron-injecting buffer2104 was formed, the second charge production region 2106 was formed onthe electron-injecting buffer 2104. Accordingly, the referencelight-emitting element 2 of this example was obtained.

Table 3 below shows the element structures of the light-emitting element2 and the reference light-emitting element 2.

TABLE 3 2103 2101 2103a 2103b 2103c 2103d 2104 2105 2106 Light- ITSONPB: NPB CzPA: Alq BPhen:Li PTCBI MoOx NPB emitting 110 nm MoOx 10 nmDPABPA 10 nm (=1:0.02) 3 nm 20 nm 10 mn element 2 (=4:1) (=1:0.1) 10 nm50 nm 30 nm Reference ITSO NPB: NPB CzPA: Alq BPhen:Li — MoOx NPB light-110 nm MoOx 10 mn DPABPA 10 nm (=1:0.02) 20 nm 10 nm emitting (=4:1)(=1:0.1) 10 nm element 2 50 nm 30 nm 2108 2108a 2108b 2108c 2102 Light-Alq:DCJTI Alq BPhen LiF Al emitting (=1:0.01) 10 nm 20 nm 1 nm 200 nmelement 2 40 nm Reference Alq:DCJTI Alq BPhen LiF Al light- (=1:0.01) 10nm 20 nm 1 nm 200 nm emitting 40 nm element 2

The thus obtained light-emitting element 2 and reference light-emittingelement 2 were sealed in a glove box under a nitrogen atmosphere so thatthey were not exposed to atmospheric air. After that, the operatingcharacteristics of these light-emitting elements were measured. Themeasurement was carried out at room temperature (under an atmosphere inwhich the temperature was kept at 25° C.).

FIG. 13 shows voltage-luminance characteristics of the light-emittingelement 2 and the reference light-emitting element 2. In FIG. 13, thehorizontal axis represents applied voltage (V) and the vertical axisrepresents luminance (cd/m²). In addition, FIG. 14 shows currentdensity-luminance characteristics. In FIG. 14, the horizontal axisrepresents voltage (V) and the vertical axis represents current density(mA/cm²). Moreover, Table 4 shows initial values of main characteristicsof the light-emitting element 2 and the reference light-emitting element2 at around 1000 cd/m².

TABLE 4 Voltage Chromaticity Current efficiency (V) (x, y) (cd/A)Light-emitting 12 (0.31, 0.27) 3 element 2 Reference 13 (0.31, 0.29) 3.8light-emitting element 2

Note that, as seen from CIE chromaticity coordinates of Table 4, thelight-emitting element 2 and the reference light-emitting element 2 bothexhibit white light emission. That is because blue-green light emissionderived from DPABPA contained in the first EL layer 2103 and red lightemission derived from DCJTI contained in the second EL layer 2108 wereboth obtained.

According to FIG. 13, the light-emitting element 2 in which theelectron-relay layer is provided can have higher luminance than thereference light-emitting element 2 when the same voltage is applied tothese light-emitting elements. In addition, according to FIG. 14, thelight-emitting element 2 has a higher current density than the referencelight-emitting element 2.

Accordingly, it was confirmed that the light-emitting element 2 of thisexample had characteristics as a light-emitting element and functionedwell. In addition, it was confirmed that the light-emitting element 2was a light-emitting element capable of being driven at low voltage.

Example 3

In Example 3, a light-emitting element that is one embodiment of thepresent invention will be described with reference to FIGS. 9A and 9B.Note that as for a light-emitting element and a reference light-emittingelement described in this example, the same or similar parts as/to orparts having the same or similar functions as/to those described inabove Examples are denoted by the same reference numerals as those ofabove Examples, and the description of them will not be repeated.

Hereinafter, a method for manufacturing a light-emitting element 3 and areference light-emitting element 3 of this example will be described.

First, the light-emitting element 3 will be described (see FIG. 9A). Thelight-emitting element 3 of this example was manufactured in a mannersimilar to that of the light-emitting element 1 described in Example 1,except for the electron-transporting layer 2103 d of the first EL layer2103 and the electron-injecting buffer 2104. As for the light-emittingelement 3 of this example, Alq with a thickness of 10 nm and BPhen witha thickness of 10 nm were stacked on the light-emitting layer 2103 c toform the electron-transporting layer 2103 d.

Next, lithium oxide (LiO₂) was evaporated to a thickness of 0.1 nm onthe electron-transporting layer 2103 d to form the electron-injectingbuffer 2104. Accordingly, the light-emitting element 3 of this examplewas obtained.

Next, the reference light-emitting element 3 will be described (see FIG.9B). The reference light-emitting element 3 of this example has thestructure of the light-emitting element 3, from which the electron-relaylayer 2105 is removed. The other layers were formed by manufacturingmethods similar to those of the light-emitting element 3. As for thereference light-emitting element 3, after the electron-injecting buffer2104 was formed, the second charge production region 2106 was formed onthe electron-injecting buffer 2104. Accordingly, the referencelight-emitting element 3 of this example was obtained.

Table 5 below shows the element structures of the light-emitting element3 and the reference light-emitting element 3.

TABLE 5 2103 2101 2103a 2103b 2103c 2103d 2104 2105 2106 Light- ITSONPB: NPB CzPA: Alq BPhen Li₂O PTCBI NPB: emitting 110 nm MoOx 10 nmDPABPA 10 nm 10 nm 0.1 nm 3 nm MoOx element 3 (=4:1) (=1:0.1) (=4:1) 50nm 30 nm 20 nm Reference ITSO NPB: NPB CzPA: Alq BPhen Li₂O — NPB:light- 110 nm MoOx 10 nm DPABPA 10 nm 10 nm 0.1 nm MoOx emitting (=4:1)(=1:0.1) (=4:1) element 3 50 nm 30 nm 20 nm 2107 2107a 2107b 2107c 2107d2102 Light- NPB Alq:DCJTI Alq BPhen LiF Al emitting 10 nm (=1:0.01) 10nm 20 nm 1 nm 200 nm element 3 40 nm Reference NPB Alq:DCJTI Alq BPhenLiF Al light- 10 nm (=1:0.01) 10 nm 20 nm 1 nm 200 nm emitting 40 nmelement 3

The thus obtained light-emitting element 3 and reference light-emittingelement 3 were sealed in a glove box under a nitrogen atmosphere so thatthey were not exposed to atmospheric air. After that, the operatingcharacteristics of these light-emitting elements were measured. Themeasurement was carried out at room temperature (under an atmosphere inwhich the temperature was kept at 25° C.).

FIG. 15 shows voltage-luminance characteristics of the light-emittingelement 3 and the reference light-emitting element 3. In FIG. 15, thehorizontal axis represents applied voltage (V) and the vertical axisrepresents luminance (cd/m²). In addition, FIG. 16 shows currentdensity-luminance characteristics. In FIG. 16, the horizontal axisrepresents voltage (V) and the vertical axis represents current density(mA/cm²). Moreover, Table 6 below shows initial values of maincharacteristics of the light-emitting element 3 and the referencelight-emitting element 3 at around 1000 cd/m².

TABLE 6 Current External Chromaticity efficiency quantum Voltage (V) (x,y) (cd/A) efficiency (%) Light-emitting 11 (0.31, 0.27) 3.6 2.7 element3 Reference 12 (0.31, 0.29) 4.3 3 light-emitting element 3

Note that, as seen from CIE chromaticity coordinates of Table 6, thelight-emitting element 3 and the reference light-emitting element 3 bothexhibit white light emission. That is because blue-green light emissionderived from DPABPA contained in the first EL layer 2103 and red lightemission derived from DCJTI contained in the second EL layer 2107 wereboth obtained.

According to FIG. 15, the light-emitting element 3 in which theelectron-relay layer is provided can have higher luminance than thereference light-emitting element 3 when the same voltage is applied tothese light-emitting elements. In addition, according to FIG. 16, thelight-emitting element 3 has a higher current density than the referencelight-emitting element 3.

Accordingly, it was confirmed that the light-emitting element 3 of thisexample had characteristics as a light-emitting element and functionedwell. In addition, it was confirmed that the light-emitting element 3was a light-emitting element capable of being driven at low voltage.

Example 4

In Example 4, a light-emitting element that is one embodiment of thepresent invention will be described with reference to FIGS. 12A and 12B.Note that as for a light-emitting element and a reference light-emittingelement described in this example, the same or similar parts as/to orparts having the same or similar functions as/to those described inabove Examples are denoted by the same reference numerals as those ofabove Examples, and the description of them will not be repeated.

Hereinafter, a method for manufacturing a light-emitting element 4 and areference light-emitting element 4 of this example will be described.

First, the light-emitting element 4 will be described (see FIG. 12A).The light-emitting element 4 was manufactured in a manner similar tothat of the light-emitting element 2 described in Example 2, except forthe electron-transporting layer 2103 d of the first EL layer 2103 andthe electron-injecting buffer 2104. As for the light-emitting element 4of this example, Alq with a thickness of 10 nm and BPhen with athickness of 10 nm were stacked on the light-emitting layer 2103 c toform the electron-transporting layer 2103 d.

Next, lithium oxide (LiO₂) was evaporated on the electron-transportinglayer 2103 d to a thickness of 0.1 nm to form the electron-injectingbuffer 2104. Accordingly, the light-emitting element 4 of this examplewas obtained.

Next, the reference light-emitting element 4 will be described (see FIG.12B). The reference light-emitting element 4 of this example has thestructure of the light-emitting element 4, from which the electron-relaylayer 2105 is removed. The other layers were formed by manufacturingmethods similar to those of the light-emitting element 4. As for thereference light-emitting element 4, after the electron-injecting buffer2104 was formed, the second charge production region 2106 was formed onthe electron-injecting buffer 2104. Accordingly, the referencelight-emitting element 4 of this example was obtained.

Table 7 below shows the element structures of the light-emitting element4 and the reference light-emitting element 4.

TABLE 7 2103 2101 2103a 2103b 2103c 2103d 2104 2105 2106 Light- ITSONPB: NPB CzPA: Alq BPhen Li₂O PTCBI MoOx NPB emitting 110 nm MoOx 10 nmDPABPA 10 nm 10 nm 0.1 nm 3 nm 20 nm 10 nm element 4 (=4:1) (=1:0.1) 50nm 30 nm Reference ITSO NPB: NPB CzPA: Alq BPhen Li₂O — MoOx NPB light-110 nm MoOx 10 nm DPABPA 10 nm 10 nm 0.1 nm 20 nm 10 nm emitting (=4:1)(=1:0.1) element 4 50 nm 30 nm 2108 2108a 2108b 2108c 2102 Light-Alq:DCJTI Alq BPhen LiF Al emitting (=1:0.01) 10 nm 20 nm 1 nm 200 nmelement 4 40 nm Reference Alq:DCJTI Alq BPhen LiF Al light- (=1:0.01) 10nm 20 nm 1 nm 200 nm emitting 40 nm element 4

The thus obtained light-emitting element 4 and reference light-emittingelement 4 were sealed in a glove box under a nitrogen atmosphere so thatthey were not exposed to atmospheric air. After that, the operatingcharacteristics of these light-emitting elements were measured. Themeasurement was carried out at room temperature (under an atmosphere inwhich the temperature was kept at 25° C.).

FIG. 17 shows voltage-luminance characteristics of the light-emittingelement 4 and the reference light-emitting element 4. In FIG. 17, thehorizontal axis represents applied voltage (V) and the vertical axisrepresents luminance (cd/m²). In addition, FIG. 18 shows currentdensity-luminance characteristics. In FIG. 18, the horizontal axisrepresents voltage (V) and the vertical axis represents current density(mA/cm²). Moreover, Table 8 shows initial values of main characteristicsof the light-emitting element 4 and the reference light-emitting element4 at around 1000 cd/m².

TABLE 8 Current Voltage Chromaticity efficiency External quantum (V) (x,y) (cd/A) efficiency (%) Light-emitting 11 (0.30, 0.26) 3 2.3 element 4Reference 13 (0.30, 0.28) 3.9 2.8 light-emitting element 4

Note that, as seen from CIE chromaticity coordinates of Table 8, thelight-emitting element 4 and the reference light-emitting element 4 bothexhibit white light emission. That is because blue-green light emissionderived from DPABPA contained in the first EL layer 2103 and red lightemission derived from DCJTI contained in the second EL layer 2108 wereboth obtained.

According to FIG. 17, the light-emitting element 4 in which theelectron-relay layer is provided can have higher luminance than thereference light-emitting element 4 when the same voltage is applied tothese light-emitting elements. In addition, according to FIG. 18, thelight-emitting element 4 has a higher current density than the referencelight-emitting element 4.

Accordingly, it was confirmed that the light-emitting element 4 of thisexample had characteristics as a light-emitting element and functionedwell. In addition, it was confirmed that the light-emitting element 4was a light-emitting element capable of being driven at low voltage.

Example 5

In Example 5, a light-emitting element that is one embodiment of thepresent invention will be described with reference to FIG. 9A. Note thatas for a light-emitting element and a reference light-emitting elementdescribed in this example, the same or similar parts as/to or partshaving the same or similar functions as/to those described in aboveExamples are denoted by the same reference numerals as those of aboveExamples, and the description of them will not be repeated.

Hereinafter, a method for manufacturing a light-emitting element 3 and areference light-emitting element 5 of this example will be described.

The light-emitting element 3 of this example was manufactured in amanner similar to that of the light-emitting element 3 described inExample 3 (see FIG. 9A). In addition, the reference light-emittingelement 5 of this example was manufactured in a manner similar to thatof the light-emitting element 3, except for the second charge productionregion 2106. As for the reference light-emitting element 5 of thisexample, NPB was evaporated on the electron-relay layer 2105, to athickness of 20 nm to form the second charge production region 2106 (seeFIG. 9A). Accordingly, the reference light-emitting element 5 wasobtained.

Table 9 below shows the element structures of the light-emitting element3 and the reference light-emitting element 5.

TABLE 9 2103 2101 2103a 2103b 2103c 2103d 2104 2105 2106 Light- ITSONPB: NPB CzPA: Alq BPhen Li₂O PTCBI NPB: emitting 110 nm MoOx 10 nmDPABPA 10 nm 10 nm 0.1 nm 3 nm MoOx element 3 (=4:1) (=1:0.1) (=4:1) 50nm 30 nm 20 nm Reference ITSO NPB: NPB CzPA: Alq BPhen Li₂O PTCBI NPBlight- 110 nm MoOx 10 nm DPABPA 10 nm 10 nm 0.1 nm 3 nm 20 nm emitting(=4:1) (=1:0.1) element 5 50 nm 30 nm 2107 2107a 2107b 2107c 2107d 2102Light- NPB Alq:DCJTI Alq BPhen LiF Al emitting 10 nm (=1:0.01) 10 nm 20nm 1 nm 200 nm element 3 40 nm Reference NPB Alq:DCJTI Alq BPhen LiF Allight- 10 nm (=1:0.01) 10 nm 20 nm 1 nm 200 nm emitting 40 nm element 5

The thus obtained light-emitting element 3 and reference light-emittingelement 5 were sealed in a glove box under a nitrogen atmosphere so thatthey were not exposed to atmospheric air. After that, the operatingcharacteristics of these light-emitting elements were measured. Themeasurement was carried out at room temperature (under an atmosphere inwhich the temperature was kept at 25° C.).

FIG. 19 shows voltage-luminance characteristics of the light-emittingelement 3 and the reference light-emitting element 5. In FIG. 19, thehorizontal axis represents applied voltage (V) and the vertical axisrepresents luminance (cd/m²). In addition, FIG. 20 shows currentdensity-luminance characteristics. In FIG. 20, the horizontal axisrepresents voltage (V) and the vertical axis represents current density(mA/cm²). Moreover, Table 10 below shows initial values of maincharacteristics of the light-emitting element 3 and the referencelight-emitting element 5 at around 1000 cd/m².

TABLE 10 Current Voltage Chromaticity efficiency External quantum (V)(x, y) (cd/A) efficiency (%) Light-emitting 11 (0.31, 0.27) 3.6 2.7element 3 Reference 21 (0.21, 0.21) 1.7 1.4 light-emitting element 5

Note that, although the light-emitting element 3 exhibited white lightemission as described in Example 3, while the reference light-emittingelement 5 had low spectrum intensity of red emission derived from DCJTIand exhibited blue-green emission (see CIE chromaticity coordinates ofTable 10). This indicates that holes are not easily injected into thesecond EL layer 2107 in the case where the second charge productionregion 2106 is formed of only a material having a high hole-transportingproperty (NPB in this example).

On the other hand, the second charge production region 2106 of thelight-emitting element 3 contains a material having a hole-transportingproperty (NPB in this example) and an acceptor material (molybdenumoxide in this example); thus, electrons are donated and accepted in thesecond charge production region 2106 and holes and electrons aregenerated in the second charge production region 2106. The generatedholes are easily moved over NPB by applied voltage to be injected intothe second EL layer 2107. In addition, the electrons are easily injectedinto the electron-relay layer 2105 by applied voltage to reach the firstEL layer 2103. Accordingly, as shown in FIG. 19, the light-emittingelement 3 has a higher luminance than the reference light-emittingelement 5. In addition, as shown in FIG. 20, a larger amount of currentcan be passed through the light-emitting element 3 by lower voltage thanin the case of the reference light-emitting element 5.

Accordingly, it was confirmed that the light-emitting element 3 of thisexample had characteristics as a light-emitting element and functionedwell. In addition, it was confirmed that the light-emitting element 3was a light-emitting element capable of being driven at low voltage.

Example 6

In Example 6, a light-emitting element that is one embodiment of thepresent invention will be described with reference to FIGS. 9A and 21.Note that as for a light-emitting element and a reference light-emittingelement described in this example, the same or similar parts as/to orparts having the same or similar functions as/to those described inabove Examples are denoted by the same reference numerals as those ofabove Examples, and the description of them will not be repeated.

A chemical formula of a material used in this example is shown below.Note that materials, the structural formulae of which have already beenshown are omitted.

Hereinafter, a method for manufacturing a light-emitting element 5 and areference light-emitting element 6 of this example will be described.

First, the light-emitting element 5 will be described (see FIG. 9A). Thelight-emitting element 5 of this example was manufactured in a mannersimilar to that of the light-emitting element 3 described in Example 3,except for the light-emitting layer 2103 c of the first EL layer 2103and the light-emitting layer 2107 b of the second EL layer 2107. As forthe light-emitting element 5 of this example, CzPA, coumarin 30, andN-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA) were co-evaporated to form the light-emittinglayer 2103 c with a thickness of 30 nm on the hole-transporting layer2103 b. In addition, similarly to the light-emitting layer 2103 c, CzPAand 2PCAPA were co-evaporated to form the light-emitting layer 2107 bwith a thickness of 30 nm on the hole-transporting layer 2107 a. Theweight ratio of CzPA to 2PCAPA in each of the light-emitting layer 2103c and the light-emitting layer 2107 b was adjusted to be 1:0.05(=CzPA:2PCAPA). Note that CzPA is a material having anelectron-transporting property and 2PCAPA that is a guest material is amaterial exhibiting green light emission. Accordingly, thelight-emitting element 5 of this example was obtained.

Next, the reference light-emitting element 6 will be described (see FIG.21). The reference light-emitting element 6 of this example has thestructure of the light-emitting element 5, from which the electron-relaylayer 2105, the charge production region 2106, and the second EL layer2107 are removed. The other layers were formed in a manner similar tothat or the light-emitting element 5. As illustrated in FIG. 21, thereference light-emitting element 6 has of this example a structure inwhich one EL layer is provided between a pair of electrodes.

As for the reference light-emitting element 6, after theelectron-injecting buffer 2104 was formed, the second electrode 2102 wasformed on the electron-injecting buffer 2104. Accordingly, the referencelight-emitting element 6 of this example was obtained.

Table 11 below shows the element structures of the light-emittingelement 5 and the reference light-emitting element 6.

TABLE 11 2103 2101 2103a 2103b 2103c 2103d 2104 2105 2106 Light- ITSONPB: NPB CzPA: Alq BPhen Li₂O PTCBI NPB: emitting 110 nm MoOx 10 nm2PCAPA 10 nm 10 nm 0.1 nm 3 nm MoOx element 5 (=4:1) (=1:0.5) (=4:1) 50nm 30 nm 60 nm Reference ITSO NPB: NPB CzPA: Alq BPhen Li₂O — — light-110 nm MoOx 10 nm 2PCAPA 10 nm 10 nm 0.1 nm emitting (=4:1) (=1:0.5)element 6 50 nm 30 nm 2107 2107a 2107b 2107c 2107d 2102 Light- NPBCzPA:2PCAPA Alq BPhen LiF Al emitting 10 nm (=1:0.05) 10 nm 20 nm 1 nm200 nm element 5 30 nm Reference — — — — — Al light- 200 nm emittingelement 6

The thus obtained light-emitting element 5 and reference light-emittingelement 6 were sealed in a glove box under a nitrogen atmosphere so thatthey were not exposed to atmospheric air. After that, the operatingcharacteristics of these light-emitting elements were measured. Themeasurement was carried out at room temperature (under an atmosphere inwhich the temperature was kept at 25° C.).

FIG. 22 shows voltage-luminance characteristics of the light-emittingelement 5 and the reference light-emitting element 6. In FIG. 22, thehorizontal axis represents applied voltage (V) and the vertical axisrepresents current density (mA/cm²). In addition, FIG. 23 shows currentdensity-luminance characteristics. In FIG. 23, the horizontal axisrepresents current density (mA/cm²) and the vertical axis representsluminance (cd/m²). Moreover, Table 12 below shows initial values of maincharacteristics of the light-emitting element 5 and the referencelight-emitting element 6 at around 1000 cd/m².

TABLE 12 Current Voltage Chromaticity efficiency External quantum (V)(x, y) (cd/A) efficiency (%) Light-emitting 7.2 (0.27, 0.64) 30 8.5element 5 Reference 3.8 (0.29, 0.62) 14 4.2 light-emitting element 6

According to FIG. 22, the light-emitting element 5 having two EL layersof this example can be driven by a voltage about twice as high as thevoltage by which the reference light-emitting element 6 including one ELlayer is driven, when current having the same current density as currentapplied to the reference light-emitting element 6 was applied to thelight-emitting element 5. In addition, according to FIG. 23, thelight-emitting element 5 exhibits luminance (that is current efficiency)about twice as high as the reference light-emitting element 6, whencurrent having the same current density as current applied to thereference light-emitting element 6 was applied to the light-emittingelement 5. The above indicates that the light-emitting element 5 of thisexample has little extra increase in voltage due to introduction of theelectron-injecting buffer, the electron-relay layer, and the chargeproduction region between the two EL layers.

Accordingly, it was confirmed that the light-emitting element 5 of thisexample had characteristics as a light-emitting element and functionedwell. In addition, it was confirmed that the light-emitting element 5was a light-emitting element which had little extra increase in voltagedue to introduction of the electron-injecting buffer, the electron-relaylayer, and the charge production region between the two EL layers, andwhich could be driven at low voltage.

Example 7

In Example 7, a light-emitting element that is one embodiment of thepresent invention will be described with reference to FIGS. 9A and 9B.Note that as for a light-emitting element and a reference light-emittingelement described in this example, the same or similar parts as/to orparts having the same or similar functions as/to those described in theabove examples are denoted by the same reference numerals as those ofthe above examples, and the description of them will not be repeated.

A chemical formula of a material used in this example is shown below.Note that materials, the structural formulae of which have already beenshown are omitted.

Hereinafter, a method for manufacturing a light-emitting element 6 and areference light-emitting element 7 of this example will be described.

First, the light-emitting element 6 will be described (see FIG. 9A). Thelight-emitting element 6 of this example was manufactured in a mannersimilar to that of the light-emitting element 5 described in Example 6,except for the electron-relay layer 2105. As for the light-emittingelement 6 of this example,N,N′-dihexyl-3,4,9,10-perylenetetracarboxylicdiimide (abbreviation:HexPTC) was evaporated to form the electron-relay layer 2105 with athickness of 3 nm on the electron-injecting buffer 2104. Accordingly,the light-emitting element 6 of this example was obtained.

Next, the reference light-emitting element 7 will be described (see FIG.9B). The reference light-emitting element 7 of this example has thestructure of the light-emitting element 6, from which the electron-relaylayer 2105 is removed. The other layers were formed by manufacturingmethods similar to those of the light-emitting element 6. As for thereference light-emitting element 7, after the electron-injecting buffer2104 was formed, the charge production region 2106 was formed on theelectron-injecting buffer 2104. Accordingly, the referencelight-emitting element 7 of this example was obtained.

Table 13 below shows the element structures of the light-emittingelement 6 and the reference light-emitting element 7.

TABLE 13 2103a 2101 2103a 2103b 2103c 2103d 2104 2105 2106 Light- ITSONPB: NPB CzPA: Alq BPhen Li₂O HexPTC NPB: emitting 110 nm MoOx 10 nm2PCAPA 10 nm 20 nm 0.1 nm 3 nm MoOx element 6 (=4:1) (=1:0.05) (=4:1) 50nm 30 nm 60 nm Reference ITSO NPB: NPB CzPA: Alq BPhen Li₂O — NPB:light- 110 nm MoOx 10 mn 2PCAPA 10 nm 20 nm 0.1 nm MoOx emitting (=4:1)(=1:0.05) (=4:1) element 7 50 nm 30 nm 60 nm 2107 2107a 2107b 2107c2107d 2102 Light- NPB CzPA: Alq BPhen LiF Al emitting 10 nm 2PCAPA 10 nm20 nm 1 nm 200 nm element 6 (=1:0.05) 30 nm Reference NPB CzPA: AlqBPhen LiF Al light- 10 nm 2PCAPA 10 nm 20 nm 1 nm 200 nm emitting(=1:0.05) element 7 30 nm

The thus obtained light-emitting element 6 and reference light-emittingelement 7 were sealed in a glove box under a nitrogen atmosphere so thatthey were not exposed to atmospheric air. After that, the operatingcharacteristics of these light-emitting elements were measured. Themeasurement was carried out at room temperature (under an atmosphere inwhich the temperature was kept at 25° C.).

FIG. 25 shows voltage-luminance characteristics of the light-emittingelement 6 and the reference light-emitting element 7. In FIG. 25, thehorizontal axis represents applied voltage (V) and the vertical axisrepresents luminance (cd/m²). In addition, FIG. 26 shows voltage-currentdensity characteristics. In FIG. 26, the horizontal axis representsvoltage (V) and the vertical axis represents current density (mA/cm²).Moreover, Table 14 below shows initial values of main characteristics ofthe light-emitting element 6 and the reference light-emitting element 7at around 1000 cd/m².

TABLE 14 Current Voltage Chromaticity efficiency External quantum (V)(x, y) (cd/A) efficiency (%) Light-emitting 7.4 (0.25, 0.64) 31 8.9element 6 Reference 7.8 (0.25, 0.63) 30 8.9 light-emitting element 7

According to FIG. 25, the light-emitting element 6 in which theelectron-relay layer is provided can have higher luminance than thereference light-emitting element 7 when the same voltage is applied tothese light-emitting elements. In addition, according to FIG. 26, thelight-emitting element 6 has a higher current density than the referencelight-emitting element 7.

Accordingly, it was confirmed that the light-emitting element 6 of thisexample had characteristics as a light-emitting element and functionedwell. In addition, it was confirmed that the light-emitting element 6was a light-emitting element capable of being driven at low voltage.

Example 8

In Example 8, a light-emitting element that is one embodiment of thepresent invention will be described with reference to FIGS. 9A and 9B.Note that as for a light-emitting element and a reference light-emittingelement described in this example, the same or similar parts as/to orparts having the same or similar functions as/to those described in theabove examples are denoted by the same reference numerals as those ofthe above examples, and the description of them will not be repeated.

Chemical formulae of materials used in this example are shown below.Note that materials, the structural formulae of which have already beenshown are omitted.

Hereinafter, a method for manufacturing a light-emitting element 7 and areference light-emitting element 8 of this example will be described.

First, the light-emitting element 7 will be described (see FIG. 9A). Thelight-emitting element 7 of this example was manufactured in a mannersimilar to that of the light-emitting element 3 in Example 3, except forthe light-emitting layer 2103 c and the electron-transporting layer 2103d of the first EL layer 2103, the second charge production region 2106,and the light-emitting layer 2107 b and the electron-transporting layer2107 c of the second EL layer 2107.

In the light-emitting element 7 of this example,4-(10-phenyl-9-anthryl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBAPA) and4-(1-naphthyl)-4′(9-phenyl-9H-carbazol-3-yl)-triphenylamine(abbreviation: PCBANB) were co-evaporated to a thickness of 20 nm, andfurthermore CzPA and SD1 (product name; manufactured by SFC Co., Ltd)were co-evaporated thereon to a thickness of 30 nm, whereby thelight-emitting layer 2103 c was formed. Note that the weight ratio ofPCBAPA to PCBANB was adjusted to be 1:1 (=PCBAPA:PCBANB). Note also thatthe weight ratio of CzPA to SD1 was adjusted to be 1:0.05 (=CzPA:SD1).

Then, BPhen was evaporated on the light-emitting layer 2103 c to athickness of 30 nm to form the electron-transporting layer 2107 c.

In addition, as for the light-emitting element 7, NPB that is a materialhaving a high hole-transporting property and molybdenum(VI) oxide thatis an acceptor material were co-evaporated on the electron-relay layer2105 to form the second charge production region 2106. The thickness ofthe second charge production region 2106 was 40 nm. The weight ratio ofNPB to molybdenum(VI) oxide was adjusted to be 4:1 (=NPB:molybdenumoxide).

Moreover, in the light-emitting element 7,4-(9H-carbazol-9-yl)-4′-(5-phenyl-1,3,4-oxadiazol-2-yl)triphenylamine)(abbreviation: YGAO11) and(acetylacetonate)bis(2,3,5-triphenylpyrazinato)iridium(III)(abbreviation: Ir(tppr)₂(acac)) were co-evaporated to a thickness of 10nm, and then YGAO11 andbis(2-phenylpyridinato-N,C^(2′))iridium(acetylacetonate) (abbreviation:Ir(ppy)₂(acac)) were co-evaporated to a thickness of 20 nm, whereby thelight-emitting layer 2107 b was formed. Note that the weight ratio ofYGAO11 to Ir(tppr)₂(acac) was adjusted to be 1:0.03 (=YGAO11:Ir(tppr)₂(acac)). Note also that that the weight ratio of YGAO11 toIr(ppy)₂(acac) was adjusted to be 1:0.06 (=YGAO11: Ir(ppy)₂(acac)).

Then, BAlq with a thickness of 10 nm and then BPhen with a thickness of20 nm were stacked on the light-emitting layer 2107 b by evaporation toform the electron-transporting layer 2107 c. Accordingly, thelight-emitting element 7 of this example was obtained.

Next, the reference light-emitting element 8 will be described (see FIG.9B). The reference light-emitting element 8 of this example has thestructure of the light-emitting element 7, from which the electron-relaylayer 2105 is removed. In addition, as for the reference light-emittingelement 8, BPhen and lithium (Li) were co-evaporated to a thickness of20 nm to form the electron-injecting buffer 2104. Here, the weight ratioof BPhen to Li was adjusted to 1:0.02 (=BPhen:Li). The other layers wereformed in manufacturing methods similar to those of the light-emittingelement 7. As for the reference light-emitting element 8, after theelectron-injecting buffer 2104 was formed, the charge production region2106 was formed on the electron-injecting buffer 2104. Accordingly, thereference light-emitting element 8 was obtained.

Table 15 below shows the element structures of the light-emittingelement 7 and the reference light-emitting element 8.

TABLE 15 2103 2101 2103a 2103b 2103c 2103d 2101 2105 2106 Light- ITSONPB: NPB PCBAPA: CzPA:SDI BPhen Li₂O PTCBI NPB: emitting 110 nm MoOx 10nm PCBANB (=1:0.05) 30 nm 0.1 nm 3 nm MoOx element 7 (=4:1) (=1:1) 20 nm(=4:1) 50 nm 20 nm 40 nm Reference ITSO NPB: NPB PCBAPA: CzPA:SDI BPhenBPhen:Li — NPB: light- 110 nm MoOx 10 mn PCBANB (=1:0.05) 10 nm(=1:0.02) MoOx emitting (=4:1) (=1:1) 30 nm 20 nm (=4:1) element 8 50 nm20 nm 40 nm 2107 2107a 2107b 2107c 2107d 2102 Light- NPB YGAO11: YGAO11:BAlq BPhen LiF Al emitting 10 nm Ir(ppr)₂acac Ir(ppy)₂acac 10 nm 20 nm 1nm 200 nm element 7 (=1:0.03) (=1:0.06) 10 nm 20 nm Reference NPBYGAO11: YGAO11: BAlq BPhen LiF Al light- 10 nm Ir(ppr)₂acac Ir(ppy)₂acac10 nm 20 nm 1 nm 200 nm emitting (=1:0.03) (=1:0.06) element 8 10 nm 20nm

The thus obtained light-emitting element 7 and reference light-emittingelement 8 were sealed in a glove box under a nitrogen atmosphere so thatthey were not exposed to atmospheric air. After that, the operatingcharacteristics of these light-emitting elements were measured. Themeasurement was carried out at room temperature (under an atmosphere inwhich the temperature was kept at 25° C.).

FIG. 27 shows voltage-luminance characteristics of the light-emittingelement 7 and the reference light-emitting element 8. In FIG. 27, thehorizontal axis represents applied voltage (V) and the vertical axisrepresents luminance (cd/m²). In addition, FIG. 28 shows voltage-currentdensity characteristics. In FIG. 28, the horizontal axis representsvoltage (V) and the vertical axis represents current density (mA/cm²).Moreover, Table 16 below shows initial values of main characteristics ofthe light-emitting element 7 and the reference light-emitting element 8at around 1000 cd/m².

TABLE 16 Current Voltage Chromaticity efficiency External quantum (V)(x, y) (cd/A) efficiency (%) Light-emitting 6.4 (0.32, 0.43) 57 23element 7 Reference 7.6 (0.32, 0.43) 59 23 light-emitting element 8

According to FIG. 27, the light-emitting element 7 in which theelectron-relay layer is provided can have higher luminance than thereference light-emitting element 8 when the same voltage is applied tothese light-emitting elements. In addition, according to FIG. 28, thelight-emitting element 7 has a higher current density than the referencelight-emitting element 8.

Note that, as seen from CIE chromaticity coordinates of Table 16 andFIG. 29, the light-emitting element 7 and the reference light-emittingelement 8 both exhibit white light emission. That is because blue lightemission derived from PCBAPA and SD1 contained in the first EL layer2103, red light emission derived from Ir(tppr)₂(acac) contained in thesecond EL layer 2107, and green light emission derived fromIr(ppy)₂(acac) were obtained.

Accordingly, it was confirmed that the light-emitting element 7 of thisexample had characteristics as a light-emitting element and functionedwell. In addition, it was confirmed that the light-emitting element 7was a light-emitting element capable of being driven at low voltage.Moreover, it was found that one embodiment of the structure of thepresent invention was effective even in the case where the structure isapplied to a white light-emitting element in which EL layers exhibitdifferent emission spectra.

Example 9

In Example 9, a light-emitting element that is one embodiment of thepresent invention will be described with reference to FIGS. 9A and 9B.Note that as for a light-emitting element and a reference light-emittingelement described in this example, the same or similar parts as/to orparts having the same or similar functions as/to those described in theabove examples are denoted by the same reference numerals as those ofthe above examples, and the description of them will not be repeated.

Chemical formulae of materials used in this example are shown below.Note that materials, the structural formulae of which have already beenshown are omitted.

Hereinafter, a method for manufacturing a light-emitting element 8 and areference light-emitting element 9 of this example will be described.

First, the light-emitting element 8 will be described (see FIG. 9A). Thelight-emitting element 8 of this example was manufactured in a mannersimilar to that of the light-emitting element in Example 8, except forthe light-emitting layer 2103 c of the first EL layer 2103, the secondcharge production region 2106, and the light-emitting layer 2107 b andthe electron-transporting layer 2107 c of the second EL layer 2107.

As for the light-emitting element 8 of this example,2,3-bis{4-[N-(4-biphenyl)-N-phenylamino]phenyl}quinoxaline(abbreviation: BPAPQ) and(acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III)(abbreviation: Ir(Fdpq)₂(acac)) were co-evaporated to a thickness of 10nm; NPB andN,9-diphenyl-N-[4-(9,10-diphenyl-2-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: 2PCAPPA) were co-evaporated to a thickness of 5 nm; andthen CzPA and 2PCAPPA were co-evaporated to a thickness of 30 nm,whereby the light-emitting layer 2103 c was formed.

Note that the weight ratio of BPAPQ to Ir(Fdpq)₂(acac) was adjusted tobe 1:0.06 (=BPAPQ:Ir(Fdpq)₂(acac)). Note also that the weight ratio ofNPB to 2PCAPPA was adjusted to be 1:0.1 (=NPB:2PCAPPA). Note also thatthe weight ratio of CzPA to 2PCAPPA was adjusted to be 1:0.1(=CzPA:2PCAPPA).

In addition, in the light-emitting element 7, NPB that is a materialhaving a high hole-transporting property and molybdenum(VI) oxide thatis an acceptor material were co-evaporated on the electron-relay layer2105 to form the second charge production region 2106. The thickness ofthe second charge production region 2106 was 70 nm. The weight ratio ofNPB to molybdenum(VI) oxide was adjusted to be 4:1 (=NPB:molybdenumoxide).

Moreover, as for the light-emitting element 7, NPB and rubrene wereco-evaporated to a thickness of 20 nm; then9-phenyl-9′-[4-(10-phenyl-9-anthryl)phenyl]-3,3′-bi(9H-carbazole)(abbreviation: PCCPA) andN,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine(abbreviation: YGA2S) were co-evaporated to a thickness of 10 nm; andfurthermore, CzPA and YGA2S were co-evaporated to a thickness of 20 nm,whereby the light-emitting layer 2107 b was formed. Note that the weightratio of NPB to rubrene was adjusted to be 1:0.015 (=NPB:rubrene). Notealso that the weight ratio of PCCPA to YGA2S was adjusted to be 1:0.05(=PCCPA:YGA2S). Note also that the weight ratio of CzPA to YGA2S wasadjusted to be 1:0.05 (=CzPA:YGA2S).

Then, BPhen was evaporated on the light-emitting layer 2107 b to formthe electron-transporting layer 2107 c. Accordingly, the light-emittingelement 8 of this example was obtained.

Next, the reference light-emitting element 9 will be described (see FIG.9B). The reference light-emitting element 9 of this example has thestructure of the light-emitting element 8, from which the electron-relaylayer 2105 is removed. Further, as for the reference light-emittingelement 9, BPhen and lithium (Li) were co-evaporated to a thickness of20 nm to form the electron-injecting buffer 2104. Here, the weight ratioof BPhen to Li was adjusted to be 1:0.02 (=BPhen:Li). The other layerswere formed by manufacturing methods similar to those of thelight-emitting element 8. As for the light-emitting element 9, after theelectron-injecting buffer 2104 was formed, the charge production region2106 was formed on the electron-injecting buffer 2104. Accordingly, thereference light-emitting element 9 of this example was obtained.

Table 17 below shows the element structures of the light-emittingelement 8 and the reference light-emitting element 9.

TABLE 17 2103 2101 a b c d 2104 2105 2106 Light- ITSO NPB: NPB BPAPQ:NPB: CzPA: BPhen Li₂O PTCBI NPB: emitting 110 nm MoOx 10 nmIr(Fdpq)₂acac 2PCAPA 2PCAPPA 30 nm 0.1 nm 3 nm MoOx element 8 (=4:1)(=1:0.06) (=1:0.1) (=1:0.1) (=4:1) 50 nm 10 nm 5 nm 30 nm 70 nmReference ITSO NPB: NPB BPAPQ: NPB: CzPA: BPhen BPhen:Li — NPB: light-110 nm MoOx 10 mn Ir(Fdpq)₂acac 2PCAPA 2PCAPPA 10 nm (=1:0.02) MoOxemitting (=4:1) (=1:0.06) (=1:0.1) (=1:0.1) 20 nm (=4:1) element 9 50 nm10 nm 5 nm 30 nm 70 nm 2107 a b c d 2102 Light- NPB NPB: PCCPA: CzPA:BPhen LiF Al emitting 10 nm Rubrene YGA2S YGA2S 30 nm 1 nm 200 nmelement 8 (=1:0.015) (=1:0.05) (=1:0.05) 20 nm 10 nm 20 nm Reference NPBNPB: PCCPA: CzPA: BPhen LiF Al light- 10 nm Rubrene YGA2S YGA2S 30 nm 1nm 200 nm emitting (=1:0.015) (=1:0.05) (=1:0.05) element 9 20 nm 10 nm20 nm

The thus obtained light-emitting element 8 and reference light-emittingelement 9 were sealed in a glove box under a nitrogen atmosphere so thatthey were not exposed to atmospheric air. After that, the operatingcharacteristics of these light-emitting elements were measured. Themeasurement was carried out at room temperature (under an atmosphere inwhich the temperature was kept at 25° C.).

FIG. 30 shows voltage-luminance characteristics of the light-emittingelement 8 and the reference light-emitting element 9. In FIG. 30, thehorizontal axis represents applied voltage (V) and the vertical axisrepresents luminance (cd/m²). In addition, FIG. 31 shows voltage-currentdensity characteristics. In FIG. 31, the horizontal axis representsvoltage (V) and the vertical axis represents current density (mA/cm²).Moreover, Table 18 below shows initial values of main characteristics ofthe light-emitting element 8 and the reference light-emitting element 9at around 1000 cd/m².

TABLE 18 Current Voltage Chromaticity efficiency External quantum (V)(x, y) (cd/A) efficiency (%) Light-emitting 6.6 (0.31, 0.36) 15 7.0element 8 Reference 7.2 (0.31, 0.37) 15 7.0 light-emitting element 9

According to FIG. 30, the light-emitting element 8 in which theelectron-relay layer is provided can have higher luminance than thereference light-emitting element 9 when the same voltage is applied tothese light-emitting elements. In addition, according to FIG. 31, thelight-emitting element 8 has a higher current density than the referencelight-emitting element 9.

Note that, as seen from CIE chromaticity coordinates of Table 18 andFIG. 32, the light-emitting element 8 and the reference light-emittingelement 9 both exhibit white light emission. That is because red lightemission derived from Ir(Fdpq)₂(acac) contained in the first EL layer2103, blue-green light emission derived from 2PCAPPA, yellow lightemission derived from rubrene contained in the second EL layer 2107, andblue light emission derived from YGA2S were obtained. In addition, thosefour emission colors are combined, whereby color rendering index (CRI)of as high as 92 was obtained.

Accordingly, it was confirmed that the light-emitting element 8 of thisexample had characteristics as a light-emitting element and functionedwell. In addition, it was confirmed that the light-emitting element 8was a light-emitting element capable of being driven at low voltage.Moreover, it was found that one embodiment of the structure of thepresent invention was effective even in the case where the structure isapplied to a white light-emitting element in which EL layers exhibitdifferent emission spectra.

Reference Example

In this reference example, a synthesis method of the material used inthe above examples will be specifically described.

Synthesis example of4-(1-naphthyl)-4′-(9-phenyl-9H-carbazol-3-yl)-triphenylamine(abbreviation: PCBANB)

Synthesis scheme of4-(1-naphthyl)-4′-(9-phenyl-9H-carbazol-3-yl)-triphenylamine is shownbelow (A-1).

In a 50-mL three-neck flask, 1.2 g (3.0 mmol) of3-(4-bromophenyl)-9-phenyl-9H-carbazole, 0.9 g (3.0 mmol) of4-(1-naphthyl)diphenylamine, 0.5 g (5.0 mmol) of sodium tert-butoxide,and 6.0 mg (0.01 mmol) of bis(dibenzylideneacetone)palladium(0) wereput, and 15 mL of dehydrated xylene was added to this mixture. Thismixture was deaerated while being stirred under low pressure. After thedeaeration, 0.06 mL (0.03 mmol) of tri(tert-butyl)phosphine (10 wt %hexane solution) was added thereto. This mixture was stirred under anitrogen atmosphere at 120° C. for 4.5 hours to be reacted.

After the reaction, 250 mL of toluene was added to this reactionmixture, and this suspension Was filtrated through Florisil, silica gel,alumina, and then Celite. The obtained filtrate was washed with water.Then, magnesium sulfate was added to remove moisture. This suspensionwas filtrated through Florisil, alumina, silica gel, and then Celite toobtain filtrate. The obtained filtrate was concentrated, and acetone andmethanol were added thereto. The mixture was exposed to ultrasonic wavesand recrystallized to obtain 1.5 g of an objective white powder at ayield of 82%.

An Rf value of the objective substance by a silica gel thin layerchromatography (TLC) (developing solvent, ethyl acetate:hexane=1:10) was0.34, that of 3-(4-bromophenyl)-9-phenyl-9H-carbazole was 0.46, and thatof 4-(1-naphthyl)diphenylamine was 0.25.

A compound that was obtained through the above step was measured by anuclear magnetic resonance method (¹H NMR). The measurement data areshown below. The measurement results show that PCBANB that was anobjective material was obtained.

¹H NMR (CDCl₃, 300 MHz): δ (ppm)=7.07 (t, J=6.6 Hz, 1H), 7.25-7.67 (m,26H), 7.84 (d, J=7.8 Hz, 1H), 7.89-7.92 (m, 1H), 8.03-8.07 (m, 1H), 8.18(d, J=7.8 Hz, 1H), 8.35 (d, J=0.9 Hz, 1H).

This application is based on Japanese Patent Application serial no.2008-306425 filed with Japan Patent Office on Dec. 1, 2008, and JapanesePatent Application serial no. 2009-131518 filed Japan Patent Office onMay 29, 2009, the entire contents of which are hereby incorporated byreference.

1. A light emitting element comprising: an anode; a first EL layer overthe anode; a first layer over the first EL layer; a second layer overand in contact with the first layer; a region over and in contact withthe second layer; a second EL layer over the region; and a cathode overthe second EL layer, wherein the first layer includes at least oneselected from the group consisting of an alkali metal, an alkaline earthmetal, a rare earth metal, an alkali metal compound, an alkaline earthmetal compound, and a rare earth metal compound, wherein the secondlayer includes a material having an electron-transporting property, andwherein the region includes a material having a hole-transportingproperty and an acceptor material.
 2. The light emitting elementaccording to claim 1, wherein a mass ratio of the acceptor material tothe material having the hole-transporting property in the region is from0.1:1 to 4.0:1.
 3. The light emitting element according to claim 1,wherein the acceptor material is a transition metal oxide which is anoxide of a metal belonging to Groups 4 to 8 of the periodic table. 4.The light emitting element according to claim 3, wherein the transitionmetal oxide is a molybdenum oxide.
 5. The light emitting elementaccording to claim 1, wherein the region further comprises7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane and/or chloranil.6. The light emitting element according to claim 1, wherein an emissionspectrum of the first EL layer is different from an emission spectrum ofthe second EL layer.
 7. The light emitting element according to claim 1,wherein the first EL layer includes an organic light emitting layer. 8.Any one of a light emitting device, a lighting device, and an electronicdevice selected from the group consisting of a camera, a goggle typedisplay, a navigation system, an audio replay device, a computer, a gamemachine, a portable information terminal device, an electronic book, acellular phone, an image replay device, and a television includes thelight emitting element according to claim
 1. 9. A light emitting elementcomprising: an anode; a first EL layer over the anode; a first layerover the first EL layer; a second layer over and in contact with thefirst layer; a region over and in contact with the second layer; asecond EL layer over the region; and a cathode over the second EL layer,wherein the first layer includes at least one selected from the groupconsisting of an alkali metal, an alkaline earth metal, a rare earthmetal, an alkali metal compound, an alkaline earth metal compound, and arare earth metal compound, wherein the second layer is configured totransport an electron from the region to the first layer, and whereinthe region includes a material having a hole-transporting property andan acceptor material.
 10. The light emitting element according to claim9, wherein a mass ratio of the acceptor material to the material havingthe hole-transporting property in the region is from 0.1:1 to 4.0:1. 11.The light emitting element according to claim 9, wherein the acceptormaterial is a transition metal oxide which is an oxide of a metalbelonging to Groups 4 to 8 of the periodic table.
 12. The light emittingelement according to claim 11, wherein the transition metal oxide is amolybdenum oxide.
 13. The light emitting element according to claim 9,wherein the region further comprises7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane and/or chloranil.14. The light emitting element according to claim 9, wherein an emissionspectrum of the first EL layer is different from an emission spectrum ofthe second EL layer.
 15. The light emitting element according to claim9, wherein the first EL layer includes an organic light emitting layer.16. Any one of a light emitting device, a lighting device, and anelectronic device selected from the group consisting of a camera, agoggle type display, a navigation system, an audio replay device, acomputer, a game machine, a portable information terminal device, anelectronic book, a cellular phone, an image replay device, and atelevision includes the light emitting element according to claim
 9. 17.A light emitting element comprising: an anode; a first EL layer over theanode; a first layer over the first EL layer; a second layer over and incontact with the first layer; a third layer over and in contact with thesecond layer; a second EL layer over the third layer; and a cathode overthe second EL layer, wherein the first layer includes at least oneselected from the group consisting of an alkali metal, an alkaline earthmetal, a rare earth metal, an alkali metal compound, an alkaline earthmetal compound, and a rare earth metal compound, wherein the secondlayer includes a material having an electron-transporting property, andwherein the third layer is a layer in which a first region including thematerial having a hole-transporting property and a second regionincluding an acceptor material are stacked.
 18. The light emittingelement according to claim 17, wherein a mass ratio of the acceptormaterial to the material having the hole-transporting property in thethird layer is from 0.1:1 to 4.0:1.
 19. The light emitting elementaccording to claim 17, wherein the acceptor material is a transitionmetal oxide which is an oxide of a metal belonging to Groups 4 to 8 ofthe periodic table.
 20. The light emitting element according to claim19, wherein the transition metal oxide is a molybdenum oxide.
 21. Thelight emitting element according to claim 17, wherein the third layerfurther comprises 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethaneand/or chloranil.
 22. The light emitting element according to claim 17,wherein an emission spectrum of the first EL layer is different from anemission spectrum of the second EL layer.
 23. The light emitting elementaccording to claim 17, wherein the first EL layer includes an organiclight emitting layer.
 24. Any one of a light emitting device, a lightingdevice, and an electronic device selected from the group consisting of acamera, a goggle type display, a navigation system, an audio replaydevice, a computer, a game machine, a portable information terminaldevice, an electronic book, a cellular phone, an image replay device,and a television includes the light emitting element according to claim17.
 25. A light emitting element comprising: an anode; a first EL layerover the anode; a first layer over the first EL layer; a second layerover and in contact with the first layer; a third layer over and incontact with the second layer; a second EL layer over the third layer;and a cathode over the second EL layer, wherein the first layer includesat least one selected from the group consisting of an alkali metal, analkaline earth metal, a rare earth metal, an alkali metal compound, analkaline earth metal compound, and a rare earth metal compound, whereinthe second layer is configured to transport an electron from the thirdlayer to the first layer, and wherein the third layer is a layer inwhich a first region including the material having a hole-transportingproperty and a second region including an acceptor material are stacked.26. The light emitting element according to claim 25, wherein a massratio of the acceptor material to the material having thehole-transporting property in the third layer is from 0.1:1 to 4.0:1.27. The light emitting element according to claim 25, wherein theacceptor material is a transition metal oxide which is an oxide of ametal belonging to Groups 4 to 8 of the periodic table.
 28. The lightemitting element according to claim 27, wherein the transition metaloxide is a molybdenum oxide.
 29. The light emitting element according toclaim 25, wherein the third layer further comprises7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane and/or chloranil.30. The light emitting element according to claim 25, wherein anemission spectrum of the first EL layer is different from an emissionspectrum of the second EL layer.
 31. The light emitting elementaccording to claim 25, wherein the first EL layer includes an organiclight emitting layer.
 32. Any one of a light emitting device, a lightingdevice, and an electronic device selected from the group consisting of acamera, a goggle type display, a navigation system, an audio replaydevice, a computer, a game machine, a portable information terminaldevice, an electronic book, a cellular phone, an image replay device,and a television includes the light emitting element according to claim25.